We present an estimation-based approach to the design of adaptive IIR filters. We also use this approach to design adaptive filters when a feedback signal from the output of the adaptive filter contaminates the reference signal. We use an H, criterion to cast the problem as a nonlinear H, filtering problem, and present an approximate linear H, filtering solution. This linear filtering solution is then used to ada.pt the adaptive IIR Filter. The presentation of the proposed adaptive algorithm is done in the context of an adaptive Active Noise Cancellation (ANC) problem. Simulations are used to examine the performance of the proposed estimation-based adaptive algorithm.