As of April 21st, the mobile-friendliness of a web page is considered an official ranking factor for Google. The search engine responded with this update on the development that more and more users use mobile devices to surf the internet
In order to keep on presenting relevant mobile results to its users, Google recommends fixing problems concerning mobile usability as quickly as possible. Anyone who still doesn’t have a mobile optimized website risks losing ranks in the SERPs.
OnPage.org has compiled the most important tips on the Google Mobile update in this infographic: