More specifically, OS X 10.5 Leopard is the last version of OS X that will run on G5 Macs, and it will run on almost all G4 Macs as well, although you will need to hack the installer to get it on a system slower than 867 MHz. They will only run on Intel x86 hardware. With the introduction of OS X 10.6 Snow Leopard in August 2009, Apple removed all support for PowerPC Macs from its operating system.
