The technique of quantum well intermixing (QWI) involves intentionally causing interdiffusion between atoms in the quantum wells (QWs) and atoms in the barriers of a QW heterostructure. This interdiffusion alters the compositional profile of the QW, which changes the quantum confinement potential, and which subsequently blueshifts the bandgap and lowers the refractive index of the QW active region. There are several different methods of QWI, all of which allow one to interdiffuse selected regions of a wafer, such that devices with different bandgaps can be monolithically integrated. QWI has been touted as an invaluable tool for the integration of photonic devices. QWI of InP-based heterostructures has been studied since the early 1990s in academia, and it has more recently made a transition into a manufacturing setting, with at least three companies currently pursuing it (Nortel Networks, Intense Photonics, and DenseLight). This paper will discuss some of the key aspects of QWI in a manufacturing process, including a critical review of different QWI techniques, and discussion of realistic device applications.