Galaxies are generically observed to host supermassive black holes(SMBHs). According to hierarchical models of galaxy formation the SMBHs must to coalesce in every or at least some galaxy merger events. However, it is still uncertain how the SMBH binary evolves after its semi-major axis has reached a few parsecs scale. There are competing mechanisms by which the SMBH binary can reduce or increase its angular momentum and therefore its merger timescale. I will discuss how these mechanisms, together with predictions of dark matter halos merger rates in a LCDM universe, can be used to constrain the rate of SMBH binaries merger conecting them with the corresponding observational signatures.