Light sources capable of emitting multiple wavelengths are key components for the implementation of wavelength division multiplexing (WDM) systems. Over the past several years, there has been remarkable progress in developing such sources. Today's WDM sources include solitary distributed feedback lasers selected by operation wavelength, tunable singlemode lasers, multi-wavelength laser arrays, and various integrated laser devices made with different technologies. This paper presents an overview of present WDM lasers, and discusses the pros and cons of the different fabrication technologies in conjunction with device performance, manufacturability, reliability and overall cost. Commercial availability of the lasers will also be reviewed.