Wavelength division multiplexing (WDM) promises to add an important new degree of freedom to optical communications. Potential applications include higher-capacity point-to-point transmission, λ-based switching systems, and a variety of new networking and routing architectures. However, the practical deployment of WDM presents major challenges to optical source technology. Depending on the application, optical terminals may be expected to provide rapid wavelength tuning or selection, or to independently provide extraordinary wavelength precision and stability without any system-level reference. Many potential WDM sources are being considered, including wavelength-selected fixed-frequency DFB lasers, a large variety of monolithic tunable semiconductor laser designs, multi-λ semiconductor DFB/DBR laser arrays, hybrid external cavity semiconductor lasers, and grating-based Er-fiber lasers. Issues that will impact the practicality of these options include long-term wavelength stability and reliability, wavelength switching range and speed, and manufacturing costs. This tutorial will describe the various approaches, and explore the pros and cons of each with emphasis on fundamental physical design limitations and practical fabrication issues that could affect both performance and cost.