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The Rayleigh-Schrodinger perturbation formalism is extended to the case of a model space, which is not necessarily degenerate. The model space defines the zero-order or model wavefunction, and the new formalism makes it possible to use a model wavefunction of multi-configurational type. The effect of the states outside the model space are as usual taken into account by means of a perturbation expansion and expressed in terms of an 'effective' Hamiltonian, operating only within the model space. The extended Rayleigh-Schrodinger formalism is used to prove the linked-diagram theorem for a multi-configurational model space in a simple way. Alternatively, this pfoblem can be handled by means of the well known formalism for degenerate perturbation, treating the splitting within the model space as due to an additional perturbation. The present approach, however, is more direct and the model space splitting is handled without summing any infinite series. The problem of convergence of the perturbation expansion is briefly discussed.

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