  This chapter on the rheology of active fluids is an attempt to correlate
theoretical and experimental work. A considerable amount of theoretical work
and most of the experimental data focus on the rheology of active fluids in a
Newtonian matrix, which displays uncommon macroscopic rheological behaviors,
such as the apparent superfluid-like state of the pusher suspensions. The
failure of the "scallop theorem" for reciprocal swimmers in a non-Newtonian
matrix is highlighted. Finally, recent findings concerning the turbulent-like
behaviour in concentrated systems are described in detail.
