A CNC machine can be programmed in different ways to machine a workpiece. In addition to creating the cutting program, many other factors also need to be considered or programmed. These include workholding devices, cutting tools, machining conditions as well as the machining strategy. The first generation CNCs were programmed manually and punched tapes were used as a medium for transferring the machine control data (MCD), that is, G-codes into a controller. Tapes were later replaced by RS232 cables, floppy disks, and finally standard computer network cables. Today’s CNC machines are controlled directly from files created by CAD/CAM or CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the component. This means that for the first time, bringing design and manufacturing under the same automation regime becomes a reachable target. Error detection features give CNC machines the ability to alert the operator in different ways including giving a ring to the operation’s mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts that are already loaded up to that tool and wait for the operator. The focus of this chapter is on a detailed account of the basics of CNC programming, and the emphasis is on G-code and Automatic Programming Tool (APT). G-code is still the dominant manual programming language for CNC machine tools. It is also the main form of control commands many CAD/CAM (or CAM) systems output. APT was developed soon after G-codes and CNC machine tools were developed to alleviate the drudgery work of straight G-code programming. Modern CAD/CAM systems these days are now becoming the main-stream tools for CNC programming.