This lecture covers the principles and applications of powder diffraction, focusing on the generation of Debye-Scherrer rings, the Scherrer equation, and the information content extracted from powder diffraction data. It explains the importance of powder diffraction in various fields such as pharmaceuticals, earth sciences, and archaeology, highlighting its ability to provide valuable insights from powder samples. The lecture also discusses common sample configurations, typical sample sizes, and the Scherrer equation for determining crystallite size. Additionally, it explores the limitations of the Scherrer equation and its unsuitability for single-crystal diffraction.