Most of our Search index is built through the work of software known as crawlers. These automatically visit publicly accessible webpages and follow links on those pages, much like you would if you were browsing content on the web. They go from page to page and store information about what they find on these pages and other publicly-accessible content in Google's Search index.

When crawlers find a webpage, our systems render the content of the page, just as a browser does. We take note of key signals — from keywords to website freshness — and we keep track of it all in the Search index.

The Google Search index contains hundreds of billions of webpages and is well over 100,000,000 gigabytes in size. It’s like the index in the back of a book — with an entry for every word seen on every webpage we index. When we index a webpage, we add it to the entries for all of the words it contains.

Our Search index contains more than just what's on the web, because helpful information can be located in other sources.

In fact, we have multiple indexes of different types of information, which is gathered through crawling, through partnerships, through data feeds being sent to us and through our own encyclopedia of facts, the Knowledge Graph.

These many indexes mean that you can search within millions of books from major libraries, find travel times from your local public transit agency, or find data from public sources like the World Bank.