In this thesis, several aspects of the optical properties of intersubband semiconductor lasers are studied theoretically, including the waveguiding properties of quantum cascade lasers (QCLs), the anticipated modulation bandwidth, gain and threshold current of intersubband lasers, and the engineering of nonlinear susceptibilities in intersubband quantum well structures. Using two computational solvers, for the Helmholtz and Schrödinger Equations respectively, optical waveguide structures and multi quantum well (MQW) structures are designed for subsequent research. The waveguide design of a QCL reported by the Bell Labs reseachers is analysed and improved upon. A four level rate equation model was used to obtain the population inversion condition and modulation response for a triple quantum well structure (TQW) designed for intersubband lasing. An analytical expression for the modulation response is first obtained, followed by a numerical computation to verify the results. It is demonstrated that there is a unique dependence of the modulation bandwidth upon the output power of the laser, and that the maximum modulation frequency does not increase monotonically with optical output power as is the case with conventional semiconductor lasers. An expression describing the optical gain of intersubband lasers is also derived. Using this, investigations into the predicted achievable gain in mid-infrared (MIR) and near-infrared (NIR) intersubband lasers are conducted. It is found that the NIR gain is at least an order of magnitude higher than that of the MIR case. Self-consistent calculations of the optical gain are also undertaken, where the rate equations and the optical gain equations are solved alternately. An intersubband structure is designed for both triple harmonic generation (THG) and four-wave mixing (FWM). The third order nonlinear susceptibilities of these respective processes in the structure were calculated and found to be comparable to those of structures designed for just one process.