Colonies of motile microorganisms, the cytoskeleton and its components,  cells and tissues have much in common with soft condensed matter systems 
(i.e. liquid crystals, amphiphiles, colloids etc.), but also exhibit behaviors that do not appear in inanimate matter and that are crucial for biological functions. These unique properties arise when the constituent  particles are active: they consume energy from internal and external sources and dissipate it by moving through the medium they inhabit.  In this talk I will give a brief introduction to the notion of "active  matter" and present some recent results on the hydrodynamics of active  nematics suspensions in two dimensions.