The defining property of an active fluid is that energy is added to the system at the small length scales of the particles that make up the fluid, instead of at the large length scales of the bounding walls or inlets of the system. Commonly studied examples include cytoplasm or its reconstituted components, collections of swimming microorganisms, and model two-dimensional layers of cells. The interplay of the energy injected at small scales and the interactions among the constituent particles leads to nonequilibrium collective behavior, including spontaneous coherent flows, sustained oscillations, active turbulence, and two-dimensional or three-dimensional topological defects in active liquid crystalline fluids. In this talk, I will give an overview of our recent and ongoing work on active fluids. The talk will focus on our theoretical and computational studies of active gels in straight and annular two-dimensional channels subject to an externally imposed shear. The gels are isotropic in the absence of externally- or activity-driven shear, but have nematic order that increases with shear rate. We determine a phase diagram of flow states, and study the stability of some of these states.