|Abstract||We present a unified approach to quantum error correction, called operator quantum error correction. Our scheme relies on a generalized notion of noiseless subsystem that is investigated here. By combining active error correction with this generalized noiseless subsystems method, we arrive at the unified approach which incorporates the known techniques –- i.e. the standard error correction model, the method of decoherence-free subspaces, and the noiseless subsystem method –- as special cases. Moreover, we demonstrate that the quantum error correction condition from the standard model is a necessary condition for all known methods of quantum error correction.