Binary systems have been established as a new class of gamma-ray emitters by the last generation of gamma-ray telescopes. This population of gamma-ray sources, with just a handful of them known so far, is split into sub-classes depending on the dominant astrophysical process that allows for particle acceleration, hence production of high- and very-high-energy photons. On one hand, we have gamma-ray binaries that, with their the pulsar winds periodically interacting with the winds from the massive companion, thus forming relativistic shocks, provide an online watch of pulsar wind nebulae. On the other hand, there are X-ray binaries driven by accretion and, as such, seen as the scale-down version of the more distant, and more massive active galactic nuclei.

In this talk I will present the main results of the field, obtained by both satellites (Fermi-LAT and Agile) and ground-based telescopes (IACTs and HAWC), and I will show how these results have shaped the state-of-the-art knowledge of these two subclasses of binaries, and, more in general, the current understanding of accretion and relativistic outflows. I will then conclude the talk showing how binaries systems provided the final proof of the link between these two main astrophysical processes: this is the case of the recently discovered millisecond transitional pulsars.