Résumé : Between simulation and experiments on real-scale testbeds, the combined use of emulation and virtualization provide a useful alternative for performing experiments on distributed systems such as clusters, grids, cloud computing or P2P systems. In this paper, we present Distem, a software tool to build distributed virtual experimental environments. Using an homogenenous set of nodes, Distem emulates a platform composed of heterogeneous nodes (in terms of number and performance of CPU cores), connected to a virtual network described using a realistic topology model. Distem relies on LXC, a low-overhead container-based virtualization solution, to achieve scalability and enable experiments with thousands of virtual nodes. Distem provides a set of user interfaces to accomodate different needs (command-line for interactive use, Ruby and REST APIs), is freely available and well documented. After a detailed description of Distem, we perform an experimental evaluation of several of its features.