This chapter looks at the principles and uses of phase change materials (PCMs) in free cooling systems as a way to lower building energy consumption. Free cooling takes advantage of the natural temperature difference between day and night to reduce the strain on heating, ventilation, and air conditioning (HVAC) systems. PCMs improve this process by storing cool energy at night and releasing it during the day, which helps shift peak cooling demand. The chapter covers the thermodynamic principles of phase change, the criteria for choosing suitable PCMs, and methods for integrating them into building components such as walls, floors, ceilings, and ventilation systems. It also presents strategies for maximizing latent heat use and overall energy savings. Case studies show how effective PCM-based free cooling can be in providing cost-effective and eco-friendly cooling solutions. The chapter ends by discussing technical challenges, including heat transfer limits and long-term stability, and suggests ways to improve system design and implementation.