A conceptually complete formalism for the quasiparticle effective masses in semiconductors is proposed. Our approach is based. on a generalized form of the theory, including the effects of the nonlocal, energy dependent electron self: energy operator Σ, which accounts for the electron-electron interaction. This introduces two -important effects on the expression of the effective mass: an explicit energy renormalization and an extra contribution to the matrix element that enters the usual . Our preliminary numerical results for prototypical GaAs show promising improvements over the results from the local density approximation for the calculated electron effective mass compared to experimental data.