Gamma-ray binaries are a new class of binary systems, discovered over the last few decades due to the progress made in gamma-ray astronomy. These sources are characterised by spectral energy distributions that are dominated by emission at gamma-ray energies, distinguishing them from other gamma-ray detected binaries. There are currently only six known systems, with a few candidate sources identified. The model most often used to explain these sources is that they consist of a non-accreting pulsar, and that the non-thermal emission originates from the shock which forms between the pulsar and stellar winds. However, a number of questions still remain regarding the exact location and production mechanism behind the emission in these binaries. Here I present a review of gamma-ray binaries, highlighting some recent achievements made, as well as some challenges that still need to be addressed, particular in the era of CTA.