This course covers the status of optical sources for DWDM systems, with emphasis on tunable lasers. It starts with semiconductor laser basics and the characteristics of DFB and DBR lasers. Tuning mechanisms and properties are described, and modified structures with extended tuning range are explained, including sampled gratings and super structure gratings. The properties of co-directional couplers in tunable lasers are discussed. Tunable external cavity lasers, tunable VCSELs, multi wavelength lasers, and non-semiconductor lasers, such as fiber lasers and waveguide lasers are described. Examples of laser structures from recent technical literature are presented. Practical aspects such as characterization, operation and control of tunable lasers are included.