Organic-inorganic hybrid perovskites are a special class of low cost semiconductors that have revolutionized the prospects for photovoltaic and optoelectronics technologies. The inorganic chemistry of this class of materials is fascinating. These compounds adopt the three-dimensional ABX3 perovskite structure, which consists of a network of corner-sharing BX6 octahedra, where the B atom is a divalent metal cation (typically Ge2+, Sn2+ or Pb2+) and X is a monovalent anion (typically Cl?, Br?, I?); the A cation is selected to balance the total charge and it can be a Cs+ or a small molecular species. Such perovskites afford several important features including excellent optical properties that are tunable by controlling the chemical compositions, they exhibit ambipolar charge transport with high mobilities. Some members exhibit long electron and hole diffusion lengths. The fundamental similarities and differences between MeNH3PbI3, MeNH3SnI3 and MeNH3GeI3 perovskites as well as other low dimensional materials will be discussed. Another class of materials gaining significance are the two-dimensional (2D) perovskites -a blend of perovskites with layered crystal structure- (Ruddlesden-Popper type) offer a greater synthetic versatility and allow for more specialized device implementation due to the directional nature of the crystal structure. A remarkable advantage of the 2D perovskites is that their functionality can be easily tuned by incorporating a wide array of organic cations into the 2D framework, in contrast to the 3D analogues which have limited scope for structural engineering.