This lecture covers the conditions required for lasing in an optical resonator, including the phase and amplitude conditions necessary for laser operation. It explains the role of gain medium, reflectivity of facets, and distributed power losses in forming a laser resonator. The lecture also discusses resonant frequencies, lasing longitudinal modes, and the emission possibilities for cavity standing waves. Additionally, it explores gain broadening, single-mode lasing, and different types of lasers such as distributed feedback lasers, external cavity lasers, and vertical cavity surface emitting lasers.