This paper proposes the application of an adaptive impedance control scheme to alleviate some of the problems associated with the presence of time delays in a haptic teleoperation system. Continuous on-line estimation of the remote environment's impedance is performed, and is then used as a local model for haptic display control. Lyapunov stability of the proposed impedance adaptation law is demonstrated. A series of experiments is performed to evaluate the performance of this teleoperation control scheme. Two performance measures are defined to assess transparency and stability of the teleoperator. Simulation results show the superior performance of the proposed adaptive scheme, with respect to direct teleoperation, particularly in terms of increasing the stability margin and of significantly ameliorating transparency in the presence of large time delays. Experimental results, using a phantom omni as the haptic master device, support this conclusion.