  Quantum key distribution (QKD) is a family of protocols for growing a private
encryption key between two parties. Despite much progress, all ground-based QKD
approaches have a distance limit due to atmospheric losses or in-fibre
attenuation. These limitations make purely ground-based systems impractical for
a global distribution network. However, the range of communication may be
extended by employing satellites equipped with high-quality optical links. This
manuscript summarizes research and development which is beginning to enable QKD
with satellites. It includes a discussion of protocols, infrastructure, and the
technical challenges involved with implementing such systems, as well as a top
level summary of on-going satellite QKD initiatives around the world.
