The recently proposed MPI Sessions extensions to the MPI standard present a new paradigm for applications to use with MPI. MPI Sessions has the potential to address several limitations of MPI’s current specification: MPI cannot be initialized within an MPI process from different application components without a priori knowledge or coordination; MPI cannot be initialized more than once; and, MPI cannot be reinitialized after MPI finalization. MPI Sessions also offers the possibility for more flexible ways for individual components of an application to express the capabilities they require from MPI at a finer granularity than is presently possible.At this time, MPI Sessions has reached sufficient maturity for implementation and evaluation, which are the focuses of this paper. This paper presents a prototype implementation of MPI Sessions, discusses certain of its performance characteristics, and describes its successful use in a large-scale production MPI application. Overall, MPI Sessions is shown to be implementable, integrable with key infrastructure, and effective, but with certain overheads involving the initialization of MPI as well as communicator construction. Small impacts on message-passing latency and throughput are noted. Open MPI was used as the implementation vehicle, but results here are also relevant to other middleware stacks.