Active galactic nuclei (AGN), supermassive black hole (SMBH) paradigm, standard model of AGN. Spectral Energy Distribution. Accretion disk physics. Thermal and non-thermal emission and variability. X-ray corona, relativistic reflection and strong gravity fields. Relativistic jets and winds/outflows. Plasma emission and absorption spectroscopy. Broad line region and narrow line region, cloud properties, Baldwin effect. Reverberation mapping and SMBH mass estimates. Cosmological framework, luminosity distance. Surveys, Eddington effect, K-correction. Selection criteria. logN-logS. Luminosity function and its evolution with cosmological redshift. Cosmic Downsizing. SMBH-galaxy co-evolution. AGN Feedback. Black hole growth by accretion and merging. Connection to gravitational waves and multimessenger astrophysics.