This document reviews the use of phase change materials (PCMs) in solar water heating systems. It discusses how PCMs can provide thermal energy storage to improve system efficiency by storing solar energy for use when sunlight is unavailable. The document examines different types of PCMs used, including paraffin and sodium acetate trihydrate. It also explores applications of PCMs in building walls, roofs, and windows to stabilize indoor temperatures. The review finds that solar water heaters equipped with PCMs can provide hot water at night, improving on conventional systems only able to heat water during the day.