The synthesis of block copolymers with controlled block lengths by atom transfer radical poly merization (ATRP) was reviewed in detail with 43 references. Three different methods were discussed: 1) synthesis of a defined molecular weight block by atom transfer radical polymerization which is isolated and subsequently used as a macroinitiator for the reinitiation by atom transfer radical polymerization of a second monomer. 2) synthesis of a macroinitiator by introduction of initiating groups onto polymers by chemical modification and use in atom transfer radical polymerization to initiate a second block. 3) combination of atom transfer radical polymerization with other living/controlled polymerizations and even conventional radical polymerization.