Multiplicity at the very low mass end of the H-R diagram

We review the properties of multiplicity of very low mass stars and brown dwarfs in different environments and at different ages. We will compare the main results of surveys performed in the field, where the objects are relatively old (1-5 Gyrs) and isolated, in the Pleiades young (similar to 120 Myr) open cluster, and in the young (similar to 5 Myr) Upper Scorpius OB association (USco). While the field and Pleiades populations seem to have very similar properties, the preliminary results obtained in Usco seem to show significant differences. If confirmed, it would mean that the phenomenons responsible for the "final" properties of multiplicity of ultracool dwarfs (spectral type later than M6) are still at work at the age of Usco, but are already over at the age of the Pleiades. We will also discuss the observed properties in the context of the predictions of the most recent models of formation and evolution.