Synchronization and synchronization manifold are of significance in nonlinear science and extensively investigated in the physical literature, but few rigorous mathematical analysis has been carried out on this problem. In this paper a concept of synchronizer is established which is more suitable in characterizing synchronization phenomena than synchronization manifold, and the existence of generalized synchronizer in unidirectionally coupled systems is proved under some conditions. (C) 2001 Elsevier Science Inc. All rights reserved.