Basic properties of A₂^VB₃^VI (where A = Bi, Sb; B = Se, Te) with the structure of tetradymite type are reviewed. Attention is paid to the properties important for the application of these materials in thermoelectric devices, i.e. electric conductivity, Seebeck coefficient, and thermal conductivity. Point defects caused by the incorporation of doping atoms into the crystals of tetradymite type are also discussed.