By the end of the 20th century SEO had firmly implanted itself as the latest profession. No longer were search engines just running spiders and programs to crawl sites, and store the collected data by topic, and serve results based on pages they had spidered. The number of documents online continued to grow expotentially, and webmasters soon realised the value of organic search listings. Search engines had to sort the vast collection of pages they had spidered and display the most relevant pages first. Early versions of search algorithms relied on webmaster provided information like meta tags, and webmasters began to abuse meta tags, causing their pages to rank for irrelevant searches. Search engines then had to develop more complex algorithms.

When Google brought a new concept to ranking web pages, called PageRank, it was for many years, the mainstay of the Google algorithm. PageRank relied heavily on incoming links, each link to a page is a vote for that page's value. The more incoming links a page had the more votes it was supposed to have, therefore deeming it a more worthy page to render. The value of each incoming link was calculated back to the PageRank of the page it was coming from and the number of outgoing links on that page. For a while PageRank proved to be sufficient at serving relevant results. It made Google the most used search engine. Their theory was that because PageRank measured the off-site factor, it would be more difficult to manipulate. Or so they thought. Webmasters soon focused on exchanging, buying, and selling links on a massive scale. Links became a dollar value.

Google, MSN, Yahoo and other search engines now look at a wider range of off-site factors. Search engines now use hundreds of factors in ranking listings on their SERPs, and they continue to change factors and thier weight. Most search engines keep their methods and ranking algorithms secret and what works and what doesn't is largely based speculation and informed (or misinformed) guesses.