This talk will provide an explanation of the G programming language commonly known as "G-code." G-code was originally developed in the 1950s to allow numerical control of industrial manufacturing equipment. G-code's major user base is not traditional programmers or software engineers, but machinists, manufacturing programmers, and those who own 3D printers. In modern times, it is used to control everything from a home-built RepRap to massive CNC milling machines to make anything you could possibly imagine.