According to Apple, Macs must be running Snow Leopard before upgrading to Lion. Lion also requires at least 2GB of RAM, and 10GB free space on the boot volume.

Lion will only run in Intel-based Macs with a Core 2 Due, Xeon, or Core i3/i5/i7 processor. This means a lot of older Intel-based Macs cannot run Lion, including first generation iMacs, MacBooks/MacBook Pros/Mac minis with Core Duo or Core Solo processors.