Modern technology uses a variety of semiconductor devices in a myriad of applications. This chapter focuses on optoelectronic devices capable of emitting and detecting light. It completes and summarizes the material by deriving the gain and rate equations for semiconductor lasers and detectors. The study proceeds along two paths. First, we start with Fermi’s golden rule. The gain curves have sharp features and not the rounding expected from the spectral broadening mechanisms. The second method uses the results for the Liouville equation, for the independent atoms. We will show why the development applies to the semiconductor laser with ‘‘vertical transitions.’’