The term 'active matter' encompasses all systems comprising entities that individually expend energy to move themselves or other objects. The statistical physics of active matter initially focused primarily on 'dry' systems in which the fluid surrounding the active particles can be neglected.

Suspensions of micro-swimmers, such as bacteria, do not fall within this framework, and their modeling poses new challenges, particularly in the dense limit (the dilute case being addressed in fluid dynamics). These dense suspensions are, however, the most common. This thesis proposes to construct continuous theories of these systems based on models of particles moving in a fluid, the only approach that avoids the arbitrariness of 'top-down' theories based on general arguments.