This book documents the development of missile and spacecraft guidance computers from the earliest efforts to the current Space Station and satellite onboard systems. This book developed from the author's presentation at the Johns Hopkins University's Applied Physics Laboratory in 2009 for the Workshop on Flight Software. The second and third editions were expanded with new material and references. The fourth edition updated the material to the state of the art in 2014, with discussions of the latest approaches and architectures, including Orion. More references wee included, and errors were corrected. This sixth edition in 2019, adds new material, and corrects some information. There is coverage of new missions and systems, particularly the emerging Cubesat architecture, and the ISS's spiffy new Supercomputer.