One of the keys to the success of League of Legends has been its accessibility to a wide range of players. It has never really demanded a beefy PC in order to run, and even its recommended spec list wasn't too oppressive.
However, now Riot has upgraded the minimum and recommended specs.
League of Legends: New Minimum and Recommended Specifications
Before you panic, take a moment -- there's not a huge difference in terms of what you'll need, and for the vast majority of players you can continue playing without even noticing.
The minimum requirements are set for a low quality setting and 1024x768 resolution, while the recommended specs raise the quality to high and the resolution to 1920x1080.
Requirements: Windows PC
Mac Requirements
Why has Riot increased the requirements?
Riot has said the decision to increase the minimum requirements for running League of Legends is the need to eliminate barriers to innovation. There have been some major losses with the changes, such as the lack of support for Windows XP and Vista, but new content requires less investment if these conditions are met, and barriers posed by outdated hardware are removed.
It will still be possible to play League of Legends without meeting the Riot Games criteria on PC, but players may have problems maintaining a stable framerate, with Riot not promising everything will work.
On Mac, however, no operating system prior to MacOS 10.12 will be able to run the client.