The nearby young σ Orionis cluster (˜360 pc, ˜3 Myr) is becoming one of the most important regions for the study of ultra low-mass star formation and its extension down to the mass regimes of the brown dwarfs and planetary-mass objects. Here, I introduce the σ Orionis cluster and present three studies that the {JOVIAN} group is developing: a pilot programme of near-infrared adaptive-optics imaging of stars of the cluster, intermediate-resolution optical spectroscopy of a large sample of stars of the cluster and a study of the mass function down to the planetary-mass domain. This paper is a summary of the content of four posters that I presented in the Ultra low-mass star formation and evolution Workshop, as single author or on behalf of different collaborations.