CLAIRE stands for Constrained Large Deformation Diffeomorphic Image Registration. It is a C/C++ software package for velocity-based diffeomorphic image registration in three dimensions. Its performance is optimized for multi-core systems. It uses MPI for data parallelism, and has been demonstrated to scale on several supercomputing platforms. CLAIRE can be executed on large-scale state-of-the-art computing systems as well as on local compute systems with limited resources.