A self-consistent field (SCF) theory and a Monte Carlo (MC) simulation technique have been developed for block copolymer systems. Calculations were carried out using both SCF theory (with a simple mean-field potential) and MC simulation for an AB diblock copolymer chain for a wide range of solvent conditions. Fairly good agreement between these two methods was found. For an ABA triblock copolymer, a sharp transition between two conformations was observed by MC simulation : in one state conformation the copolymer has two collapsed A globules at either end of the B block chain and in the other state conformation one collapsed A globule comprising of the two A blocks with a loop of the B block chain sticking out. It is our strong suspicion that this would be a true phase transition in the limit of infinite chain length.