  Gamma-ray binaries allow us to study physical processes such as particle
acceleration up to very-high energies and gamma-ray emission and absorption
with changing geometrical configurations on a periodic basis. These sources
produce outflows of radio-emitting particles whose structure can be imaged with
Very Long Baseline Interferometry (VLBI). We have studied the changing
morphology of the gamma-ray binaries LS I +61 303 and LS 5039, and we have
discovered the extended emission of PSR B1259-63 and HESS J0632+057. Based on
these results, we have established the basic properties and behaviour of the
radio emission of gamma-ray binaries on AU scales, and we have contributed to
find characteristics that are common to all of them. Here we present the most
relevant properties of each source and the general properties of gamma-ray
binaries, and we describe the implications on the nature of these binary
systems.
