I shall present a novel method to calculate the stellar mass in AGN host galaxies. The method is based on the well known properties of the "Star Forming Sequence" and the recently discovered correlation between the luminosity of AGN and the star formation rate in their hosts. Given these relationships, one can calculate M(BH)/M(galaxy) at almost all redshifts and obtain important information about the co-evolution of AGN and SF galaxies, the relative duty cycle of SF and AGN activity, and more.