Perovskites for photovoltaics: Three-dimensional hybrid organic perovskites, CH3NH3PbX3 (X = I, Br, Cl), have recently been used to strongly improve the efficiency of dye sensitized solar cells (DSSC) leading to a new class of low-cost mesoscopic solar cells. Density functional theory (DFT) and many body (GW) corrections are used by Jacky Even et al. (pp. 31–35) to compare lead and tin hybrid and all-inorganic perovskites. The room-temperature optical absorption is associated to electronic transitions between the spin–orbit split-off band in the conduction band and the valence band. The low-temperature excitonic effect is studied by comparing the RPA and the Bethe–Salpeter (BSE) computation of the dielectric constant. Spin–orbit coupling is about three times smaller for tin. The recently reported phases of CH3NH3MI3 (where M = Pb, Sn) close to the room-temperature and tetragonal distortions from the ideal cubic phase, are analysed by a k·p perturbation, including spin–orbit effect. In addition, the non-centrosymmetric phases of CH3NH3MI3 exhibit a splitting of the electronic bands away from the critical point.