In this letter we use adaptive parameter and state feedback control to synchronize two or more slightly mismatched chaotic systems. Chua's circuit with a smooth nonlinearity is used throughout to illustrate our approach. We specify the conditions under which the parameter of a slave system will automatically converge to the parameter of the master system. We also consider potential applications of the control system to problems of secure communications and synchronization of chaos in a chain of slightly different Chua's circuits.