The Global Atmosphere-ocean IN-situ observing System (GAINS) is a program to develop the systems needed for an operational, global, in-situ observing system. Observations taken from the GAINS platforms will complement existing operational systems by filling in holes over oceans, and by delivering observations that are finer temporal and spatial resolutions than are currently possible. This paper gives an overview of the GAINS program, discussing the motivation for this observing system, the concept, and the seven-year development program. Balloon trajectory simulations illustrating the feasibility of the proposed GAINS network of 400 platforms will be presented. Finally, status of the program and results of the experimental flights will be discussed. Published by Elsevier Science Ltd on behalf of COSPAR.