We revise Krein's extension theory of semi-bounded Hermitian operators by reducing the problem to finding all positive and contractive extensions of the "resolvent operator" of . Our treatment is somewhat simpler and more natural than Krein's original method which was based on the Krein transform . Apart from being positive and symmetric, we do not impose any further constraints on the operator : neither its closedness nor the density of its domain is assumed. Moreover, our arguments remain valid in both real or complex Hilbert spaces.