RepEx is a flexible Python package which is aimed at enabling users to perform Replica Exchange (RE) simulations at scale, unattainable by conventional molecular simulations software packages with RE functionality. RepEx relies on a concept of a Pilot Job to run RE simulation on HPC clusters. RepEx is using RADICAL-Pilot  pilot system for execution of it's workloads. Currently RepEx supports two MD engins as it's application kernels: NAMD  and Amber . RepEx is modular, object-oriented code, designed to facilitate development of extension modules by it's users. For more information please see RepEx documentation.