The AC loss characteristics of high Tc superconductors are reviewed. AC losses are organized according to their generating mechanism and the magnetic field components generating them. Useful analytical formulae of hysteresis losses are presented for the superconductor slab exposed to a parallel magnetic field, for the thin strip exposed to a transverse magnetic field, and for the strip and ellipse carrying transport currents. Experimental and numerical methods for evaluating AC loss are reviewed, and these methods are applied to quantify AC losses in some practical superconductors. The AC loss characteristics of single-coated conductors are shown together with the reduction of their AC losses through striation. The AC loss characteristics of mono-layer as well as multi-layer superconducting power transmission cables are explained, and reduction of their AC losses by means of using narrower coated conductors is discussed.