Recipes for ULX formation: necessary ingredients and garnishments

We summarize the main observational features that seem to recur more frequently in the ULX population. We speculate that low metal abundance, and clustered star formation triggered by molecular cloud collisions are two fundamental physical requirements for ULX formation. In this scenario, most ULXs are formed from recent stellar processes, have black hole (BH) masses <100 M_⊙, and do not require merger processes in super-star-clusters.