In the field of multi-agent based simulation (MABS), the concept of environment is omnipresent, though poorly defined. We argue here that depending on the modeling of space and of relations between agents, only a few efficient implementations can be set up. We aim at formalizing the core functions of environments, so as to highlight the computational answers to possible modeling choices. This unifying approach leads to the identification of four paradigmatic Design Patterns, associated with specific atomic environments, which can be composed in order to tackle complex situations.