This Design Guide provides the information and resources necessary to design and install cross-linked polyethylene (PEX) water supply systems in residential buildings. It includes comprehensive design concepts and installation guidelines to increase the acceptance and proper use of PEX. This document is targeted to meet the needs of home builders, designers, and trade contractors. Its purpose is to introduce potential users to PEX and to enable current users to optimize their PEX plumbing and minimize system costs. In addition, it will allow code inspectors and homeowners to become familiar with the applications, performance characteristics, and benefits of PEX water supply systems.