This thesis discusses the integration of the subsystems in a CubeSat satellite and the implementation of a central “On-Board Controller & Data Handling” system. Potential radiation problems are investigated and addressed. CubeSTAR is a CubeSat satellite developed at the University of Oslo. The satellite requires some method of handling commands and sending data to the ground station. The satellite needs to be autonomous, so a method of coordinating the operation of each subsystem is required. The On-Board Controller integrates all the satellite’s subsystems to a unified control- and monitoring system. It will make sure each subsystem performs its operation and attempts recovery if something is out of the ordinary. Space radiation can cause memory bit-flips and other effects on electronics within a satellite. In case a subsystem does not function as expected or has disruptive behavior, it will need to be recovered or disabled. The On-Board Controller addresses the memory bit-flip problem caused by space radiation by using a new memory technology (MRAM) and a special 8-bit hamming code for commands and variables.