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.