This lecture covers the requirements and functions of a telecommand system in spacecraft, including uploading programs, controlling spacecraft operations, managing subsystems, and ensuring spacecraft safety. It also discusses the telecommand chain, signal classification, transmission link windows, and packet telecommand systems. The lecture delves into the CCSDS telecommand system, data handling, and the relationship between telemetry and telecommand systems. It concludes with an overview of the SOIS standards and the architecture of spacecraft onboard interface services.