Active galactic nuclei, AGN, are galaxies with signs of non-stellar activities in their nuclei powered by accretion onto supermassive black holes, SMBHs. The metric around astrophysical black holes is described by the mass and spin of the black hole, which together with the accretion rate of matter regulate the observed properties of AGN. The radiation and the kinetic jet power of AGN can thus be used to infer the spin of SMBHs. I will present some observational constraints on the spin from AGN demographics, as well as discussing how the spin of SMBHs might explain the radio-loudness of AGN. I will show an estimate of the spin history of SMBHs and discuss the possible mechanisms regulating the spin.