In this thesis we describe the calculation of the electronic subband structures of semiconductor quantum wells and superlattices in the Envelope Function Approximation. We show how the results of these calculations demonstrate the possibility for improvement of long-wavelength semiconductor laser device characteristics by bandstructure engineering. We present calculations for proposed strained layer and (111)-grown laser structures. We describe the observation of the pressure dependence of the light-hole mass in a quantum well within the framework of our calculational scheme, perform calculations for SiGe superlattices and give analytic results for quantum well, valence band-edge, effective masses in a 2-band (heavy-hole/light-hole) model.