The progress of heterostructure technology has permitted the fabrication and study of devices which inherently employ quantum-mechanical effects in their operation [1,3,2,4]. Their existence and technological potential have created the need for effective theoretical tools to help understand, describe, predict and optimize the performance of quantum devices. The fundamental property of heterostructures, that the energy-band structure (or electron dispersion relation) varies with position, requires some modifications to the standard textbook treatments of such things as scattering amplitudes. The present chapter describes the theoretical tools appropriate for quantum-scale heterostructure devices, and attempts to place the significant approaches in an appropriate context.