  The study of thermal heat engines was pivotal to establishing the principles
of equilibrium thermodynamics, with implications far wider than only engine
optimization. For nonequilibrium systems, which by definition dissipate energy
even at rest, how to best convert such dissipation into useful work is still
largely an outstanding question, with similar potential to illuminate general
physical principles. We review recent theoretical progress in studying the
performances of engines operating with active matter, where particles are
driven by individual self-propulsion. We distinguish two main classes, either
autonomous engines exploiting a particle current, or cyclic engines applying
periodic transformation to the system, and present the strategies put forward
so far for optimization. We delineate the limitations of previous studies, and
propose some futures perspectives, with a view to building a consistent
thermodynamic framework far from equilibrium.
