The QB50 Project will demonstrate the possibility of launching a network of 50 CubeSats built by CubeSat teams from all over the world to perform first-class science and in-orbit demonstration in the largely unexplored middle and lower thermosphere. Space agencies are currently not pursuing a multi-spacecraft network for in-situ measurements in the middle and lower thermosphere because the cost of a network of 50 satellites built to industrial standards would be very high and not justifiable in view of the limited orbital lifetime. No atmospheric network mission for in-situ measurements has been carried out in the past or is planned for the future. A network of satellites for in-situ measurements in the middle and lower thermosphere can only be realised by using very low-cost satellites, and CubeSats are the only realistic option. The Project will demonstrate the sustained availability of low-cost launch opportunities, for launching small payloads into low-Earth orbit; these could be microsatellites or networks of CubeSats or nanosats or many individual small satellites for scientific or technological research. The Project will include the development of a deployment system for the deployment into orbit of a large number of single, double or triple CubeSats. Once, the system is developed for QB50 it can be easily adapted to other missions. QB50 will also provide a launch opportunity for key technology demonstration on IOD CubeSats such as propulsion systems and aerobrakes. Up to 50 CubeSats will be launched together to low earth orbit. Due to atmospheric drag, the orbits of the CubeSats will decay and progressively lower and lower layers of the thermosphere will be explored without the need for on-board propulsion, perhaps down to 200 km. QB50 will be among the first CubeSat networks in orbit