The purpose of this thesis is to develop circuit models for a variety of semiconductor injection lasers. Circuit models provide a very convenient way of describing and analysing complex mathematical systems. They are easily transportable between computers, since only a standard circuit analysis program is required. In addition, they provide an intuitive feel for the operation of the system. The models described in this thesis are based on the physics of the devices, and include electron-photon interactions in the active region, the heterojunction current-voltage characteristic, the effects of lateral carrier diffusion in the active region, and electrical parasitics associated with the package and chip. The models are synthesised as two-port circuits using standard circuit elements, with the input port representing the external connections to the device, and the voltage of the output port representing a scaled analogue of the light output power. The major emphasis of this work is on the high-frequency direct modulation characteristics of the lasers. The thesis commences by showing that a simple rate equation model of the laser dynamics is inadequate in explaining the observed high frequency modulation characteristics for a variety of lasers. Electrical parasitics and lateral carrier diffusion are identified as the major limitations to wide-band modulation. These two effects are investigated in detail, and are incorporated in new comprehensive models of the devices. Using the new models, good agreement is demonstrated between measured and modelled small-signal modulation responses up to 6 GHz. The large-signal (pulse) response of the comprehensive models is also compared with measured data, and in general, agreement between the measured and modelled results is good. The models provide insight into the details of operation of the lasers at high modulation rates, and will lead to improvements in design of the devices and act as a tool for designing high-speed optical transmitter circuits.