The paper describes a simple circuit model for semiconductor lasers in single mode operation. This model is based on theorems well established in the microwave field, and applicable to any resonant system. A particular advantage is that, through the relationship between reflection coefficient and admittance, the same circuit, consisting only of a singleLCRresonance, can be used in a totally general way for DFB and Fabry Perot structures. In addition the shape of the spectra can be examined using Fourier techniques. Results are presented for Fabry Perot lasers which show excellent agreement with established theory. DFB lasers, both uniform devices and those with a non-corrugated phase shift section, are then examined. A general method for a phase slip DFB laser with an external cavity is presented and results shown for simple cases.