JANNUS(Joint Accelerators for Nano‐Science and Nuclear Simulation) is a project designed to study the modification of materials using multiple ion beams and in‐situ TEM observation. It will be a unique facility in Europe for the study of irradiation effects, the simulation of material damage due to irradiation and in particular of combined effects. The project is also intended to bring together experimental and modelling teams for a mutual fertilisation of their activities. It will also contribute to the teaching of particle‐matter interactions and their applications.JANNUSwill be composed of three accelerators with a common experimental chamber and of two accelerators coupled to a 200 kV TEM. © 2004 American Institute of Physics