Closed-loop motion control systems use feedback to ensure the precise positioning of mechanical components. Self-contained position sensors, such as rotary encoders, can do a good job of providing this feedback. However, in some cases it is technically and economically preferable to build the sensors into the machinery being controlled, avoiding the cost and complexity of external measurement devices. This is the domain of kit (or modular) encoders – position sensor elements designed to be installed inside motor housings or other types of equipment to measure drive shaft rotation directly. This paper describes some of the technologies used for kit encoders and outlines their relative advantages and limitations.