Mixed finite element methods differ from standard finite element schemes in their use of stationary variational principles, i.e., principles which characterize the solution of the problem to be approximated as a stationary point rather than as a strict minimum or maximum. One consequence of this property is the possibility of instabilities in the approximation, and current research has emphasized the establishment of conditions which insure both convergence and stability. From a practical standpoint it is important that these conditions be as sharp as possible so that the user not be forced into needless complications to obtain stable and accurate approximations.