Today is the 20th anniversary of the robots.txt directive being available for webmasters to block search engines from crawling their pages.
The robots.txt was created by Martijn Koster in 1994 while he was working at Nexor after having issues with crawlers hitting his sites too hard. All major search engines back then, including WebCrawler, Lycos and AltaVista, quickly adopted it; and even 20 years later, all major search engines continue to support it and obey it.
Brian Ussery posted on his blog about the 20-year anniversary and documented the most common robots.txt mistakes he has seen over his SEO tenure. It is well worth scanning through, because the robots.txt, if implemented wrong, can be severely detrimental to your rankings and search marketing success.