How Search Engines Work: Crawling, Indexing

How do search engines work?

Search engines have three primary functions:

  1. Crawl: Scour the Internet for content, looking over the code/content for each URL they find.
  2. Index: Store and organize the content found during the crawling process. Once a page is in the index, it’s in the running to be displayed as a result to relevant queries.
  3. Rank: Provide the pieces of content that will best answer a searcher’s query, which means that results are ordered by most relevant to least relevant.

What is search engine crawling?

Crawling is the discovery process in which search engines send out a team of robots (known as crawlers or spiders) to find new and updated content. Content can vary — it could be a webpage, an image, a video, a PDF, etc. — but regardless of the format, content is discovered by links.

Googlebot starts out by fetching a few web pages, and then follows the links on those webpages to find new URLs. By hopping along this path of links, the crawler is able to find new content and add it to their index called Caffeine — a massive database of discovered URLs — to later be retrieved when a searcher is seeking information that the content on that URL is a good match for.

What is a search engine index?

Search engines process and store information they find in an index, a huge database of all the content they’ve discovered and deem good enough to serve up to searchers.

Search engine ranking

When someone performs a search, search engines scour their index for highly relevant content and then orders that content in the hopes of solving the searcher’s query. This ordering of search results by relevance is known as ranking. In general, you can assume that the higher a website is ranked, the more relevant the search engine believes that site is to the query.

It’s possible to block search engine crawlers from part or all of your site, or instruct search engines to avoid storing certain pages in their index. While there can be reasons for doing this, if you want your content found by searchers, you have to first make sure it’s accessible to crawlers and is indexable. Otherwise, it’s as good as invisible.

In SEO, not all search engines are equal

Many beginners wonder about the relative importance of particular search engines. Most people know that Google has the largest market share, but how important it is to optimize for Bing, Yahoo, and others? The truth is that despite the existence of more than 30 major web search engines, the SEO community really only pays attention to Google. Why? The short answer is that Google is where the vast majority of people search the web. If we include Google Images, Google Maps, and YouTube (a Google property), more than 90% of web searches happen on Google — that’s nearly 20 times Bing and Yahoo combined.

Crawling: Can search engines find your pages?

As you’ve just learned, making sure your site gets crawled and indexed is a prerequisite to showing up in the SERPs. If you already have a website, it might be a good idea to start off by seeing how many of your pages are in the index. This will yield some great insights into whether Google is crawling and finding all the pages you want it to, and none that you don’t.

One way to check your indexed pages is “”, an advanced search operator. Head to Google and type “” into the search bar. This will return results Google has in its index for the site specified:

The number of results Google displays (see “About XX results” above) isn’t exact, but it does give you a solid idea of which pages are indexed on your site and how they are currently showing up in search results.

For more accurate results, monitor and use the Index Coverage report in Google Search Console. You can sign up for a free Google Search Console account if you don’t currently have one. With this tool, you can submit sitemaps for your site and monitor how many submitted pages have actually been added to Google’s index, among other things.

If you’re not showing up anywhere in the search results, there are a few possible reasons why:

  • Your site is brand new and hasn’t been crawled yet.
  • Your site isn’t linked to from any external websites.
  • Your site’s navigation makes it hard for a robot to crawl it effectively.
  • Your site contains some basic code called crawler directives that is blocking search engines.
  • Your site has been penalized by Google for spammy tactics.

Tell search engines how to crawl your site

If you used Google Search Console or the “” advanced search operator and found that some of your important pages are missing from the index and/or some of your unimportant pages have been mistakenly indexed, there are some optimizations you can implement to better direct Googlebot how you want your web content crawled. Telling search engines how to crawl your site can give you better control of what ends up in the index.

Most people think about making sure Google can find their important pages, but it’s easy to forget that there are likely pages you don’t want Googlebot to find. These might include things like old URLs that have thin content, duplicate URLs (such as sort-and-filter parameters for e-commerce), special promo code pages, staging or test pages, and so on.

To direct Googlebot away from certain pages and sections of your site, use robots.txt.


Robots.txt files are located in the root directory of websites (ex. and suggest which parts of your site search engines should and shouldn’t crawl, as well as the speed at which they crawl your site, via specific robots.txt directives.

How Googlebot treats robots.txt files

  • If Googlebot can’t find a robots.txt file for a site, it proceeds to crawl the site.
  • If Googlebot finds a robots.txt file for a site, it will usually abide by the suggestions and proceed to crawl the site.
  • If Googlebot encounters an error while trying to access a site’s robots.txt file and can’t determine if one exists or not, it won’t crawl the site.

Optimize for crawl budget!

Crawl budget is the average number of URLs Googlebot will crawl on your site before leaving, so crawl budget optimization ensures that Googlebot isn’t wasting time crawling through your unimportant pages at risk of ignoring your important pages. Crawl budget is most important on very large sites with tens of thousands of URLs, but it’s never a bad idea to block crawlers from accessing the content you definitely don’t care about. Just make sure not to block a crawler’s access to pages you’ve added other directives on, such as canonical or noindex tags. If Googlebot is blocked from a page, it won’t be able to see the instructions on that page.

Not all web robots follow robots.txt. People with bad intentions (e.g., e-mail address scrapers) build bots that don’t follow this protocol. In fact, some bad actors use robots.txt files to find where you’ve located your private content. Although it might seem logical to block crawlers from private pages such as login and administration pages so that they don’t show up in the index, placing the location of those URLs in a publicly accessible robots.txt file also means that people with malicious intent can more easily find them. It’s better to NoIndex these pages and gate them behind a login form rather than place them in your robots.txt file.

You can read more details about this in the robots.txt portion of our Learning Center.

Defining URL parameters in GSC

Some sites (most common with e-commerce) make the same content available on multiple different URLs by appending certain parameters to URLs. If you’ve ever shopped online, you’ve likely narrowed down your search via filters. For example, you may search for “shoes” on Amazon, and then refine your search by size, color, and style. Each time you refine, the URL changes slightly:

How does Google know which version of the URL to serve to searchers? Google does a pretty good job at figuring out the representative URL on its own, but you can use the URL Parameters feature in Google Search Console to tell Google exactly how you want them to treat your pages. If you use this feature to tell Googlebot “crawl no URLs with ____ parameter,” then you’re essentially asking to hide this content from Googlebot, which could result in the removal of those pages from search results. That’s what you want if those parameters create duplicate pages, but not ideal if you want those pages to be indexed.

Can crawlers find all your important content?

Now that you know some tactics for ensuring search engine crawlers stay away from your unimportant content, let’s learn about the optimizations that can help Googlebot find your important pages.

Sometimes a search engine will be able to find parts of your site by crawling, but other pages or sections might be obscured for one reason or another. It’s important to make sure that search engines are able to discover all the content you want indexed, and not just your homepage.

Is your content hidden behind login forms?

If you require users to log in, fill out forms, or answer surveys before accessing certain content, search engines won’t see those protected pages. A crawler is definitely not going to log in.

Are you relying on search forms?

Robots cannot use search forms. Some individuals believe that if they place a search box on their site, search engines will be able to find everything that their visitors search for.

Is text hidden within non-text content?

Non-text media forms (images, video, GIFs, etc.) should not be used to display text that you wish to be indexed. While search engines are getting better at recognizing images, there’s no guarantee they will be able to read and understand it just yet. It’s always best to add text within the <HTML> markup of your webpage.

Can search engines follow your site navigation?

Just as a crawler needs to discover your site via links from other sites, it needs a path of links on your own site to guide it from page to page. If you’ve got a page you want search engines to find but it isn’t linked to from any other pages, it’s as good as invisible. Many sites make the critical mistake of structuring their navigation in ways that are inaccessible to search engines, hindering their ability to get listed in search results.

Common navigation mistakes that can keep crawlers from seeing all of your site:

  • Having a mobile navigation that shows different results than your desktop navigation
  • Any type of navigation where the menu items are not in the HTML, such as JavaScript-enabled navigations. Google has gotten much better at crawling and understanding Javascript, but it’s still not a perfect process. The more surefire way to ensure something gets found, understood, and indexed by Google is by putting it in the HTML.
  • Personalization, or showing unique navigation to a specific type of visitor versus others, could appear to be cloaking to a search engine crawler
  • Forgetting to link to a primary page on your website through your navigation — remember, links are the paths crawlers follow to new pages!

Indexing: How do search engines interpret and store your pages?

Once you’ve ensured your site has been crawled, the next order of business is to make sure it can be indexed. That’s right — just because your site can be discovered and crawled by a search engine doesn’t necessarily mean that it will be stored in their index. In the previous section on crawling, we discussed how search engines discover your web pages. The index is where your discovered pages are stored. After a crawler finds a page, the search engine renders it just like a browser would. In the process of doing so, the search engine analyzes that page’s contents. All of that information is stored in its index.

Can I see how a Googlebot crawler sees my pages?

Yes, the cached version of your page will reflect a snapshot of the last time Googlebot crawled it.

Google crawls and caches web pages at different frequencies. More established, well-known sites that post frequently like will be crawled more frequently than the much-less-famous website for Roger the Mozbot’s side hustle, (if only it were real…)

You can view what your cached version of a page looks like by clicking the drop-down arrow next to the URL in the SERP and choosing “Cached”:

You can also view the text-only version of your site to determine if your important content is being crawled and cached effectively.

Are pages ever removed from the index?

Yes, pages can be removed from the index! Some of the main reasons why a URL might be removed include:

  • The URL is returning a “not found” error (4XX) or server error (5XX) – This could be accidental (the page was moved and a 301 redirect was not set up) or intentional (the page was deleted and 404ed in order to get it removed from the index)
  • The URL had a noindex meta tag added – This tag can be added by site owners to instruct the search engine to omit the page from its index.
  • The URL has been manually penalized for violating the search engine’s Webmaster Guidelines and, as a result, was removed from the index.
  • The URL has been blocked from crawling with the addition of a password required before visitors can access the page.

If you believe that a page on your website that was previously in Google’s index is no longer showing up, you can use the URL Inspection tool to learn the status of the page, or use Fetch as Google which has a “Request Indexing” feature to submit individual URLs to the index. (Bonus: GSC’s “fetch” tool also has a “render” option that allows you to see if there are any issues with how Google is interpreting your page).

Tell search engines how to index your site

Robots meta directives

Meta directives (or “meta tags”) are instructions you can give to search engines regarding how you want your web page to be treated.

You can tell search engine crawlers things like “do not index this page in search results” or “don’t pass any link equity to any on-page links”. These instructions are executed via Robots Meta Tags in the <head> of your HTML pages (most commonly used) or via the X-Robots-Tag in the HTTP header.

Robots meta tag

The robots meta tag can be used within the <head> of the HTML of your webpage. It can exclude all or specific search engines. The following are the most common meta directives, along with what situations you might apply them in.

index/noindex tells the engines whether the page should be crawled and kept in a search engines’ index for retrieval. If you opt to use “noindex,” you’re communicating to crawlers that you want the page excluded from search results. By default, search engines assume they can index all pages, so using the “index” value is unnecessary.

  • When you might use: You might opt to mark a page as “noindex” if you’re trying to trim thin pages from Google’s index of your site (ex: user generated profile pages) but you still want them accessible to visitors.

follow/nofollow tells search engines whether links on the page should be followed or nofollowed. “Follow” results in bots following the links on your page and passing link equity through to those URLs. Or, if you elect to employ “nofollow,” the search engines will not follow or pass any link equity through to the links on the page. By default, all pages are assumed to have the “follow” attribute.

  • When you might use: nofollow is often used together with noindex when you’re trying to prevent a page from being indexed as well as prevent the crawler from following links on the page.

noarchive is used to restrict search engines from saving a cached copy of the page. By default, the engines will maintain visible copies of all pages they have indexed, accessible to searchers through the cached link in the search results.

  • When you might use: If you run an e-commerce site and your prices change regularly, you might consider the noarchive tag to prevent searchers from seeing outdated pricing.

Here’s an example of a meta robots noindex, nofollow tag:

<!DOCTYPE html>
<meta name="robots" content="noindex, nofollow" />

This example excludes all search engines from indexing the page and from following any on-page links. If you want to exclude multiple crawlers, like googlebot and bing for example, it’s okay to use multiple robot exclusion tags.

Meta directives affect indexing, not crawling

Googlebot needs to crawl your page in order to see its meta directives, so if you’re trying to prevent crawlers from accessing certain pages, meta directives are not the way to do it. Robots tags must be crawled to be respected.


The x-robots tag is used within the HTTP header of your URL, providing more flexibility and functionality than meta tags if you want to block search engines at scale because you can use regular expressions, block non-HTML files, and apply sitewide noindex tags.

For example, you could easily exclude entire folders or file types (like

<Files ~ “\/?no\-bake\/.*”>
 Header set X-Robots-Tag “noindex, nofollow”

The derivatives used in a robots meta tag can also be used in an X-Robots-Tag.

Or specific file types (like PDFs):

<Files ~ “\.pdf$”>
 Header set X-Robots-Tag “noindex, nofollow”

For more information on Meta Robot Tags, explore Google’s Robots Meta Tag Specifications.