This project consists of a literature survey followed by the proposal, development and test, in simulation, of an extension of the traditional Guidance, Control and Navigation loop for a single spacecraft to a set of spacecraft flying in formation. This extension creates some novel challenges, since each spacecraft can be considered an obstacle for its team-mates, especially during formation initialisation, relative or absolute information about the formation state can be considered, both state estimation and control can be centralized/distributed or decentralized and do not necessarily need to be tied to the actual topology of the spacecraft formation. A study will be carried out so as to identify different approaches to the state estimation and control of spacecraft formations, as well as to compare them according to different criteria such as fuel consumption, fuel distribution across the spacecraft, robustness to spacecraft failure, communication link failure, individual sensor failure or temporary occlusions of either communications or sensor reading. Decentralized solutions seem to be the most promising approach, as they do not depend on a communications link and/or on a central spacecraft. As such, this work will devote more attention to solutions where both estimation and control can be computed locally at each spacecraft and thus depend solely on relative measurements, feasible at each spacecraft. Nevertheless, centralized/distributed approaches will be covered as well, and issues such as robustness in the presence of temporary occlusion or permanent failure of sensors and communications, as well as the minimization of information flow will arise.