Chlorinated polyvinyl chloride (CPVC) is increasingly replacing steel pipe in fire protection systems due to lower cost and ease of installation. However, CPVC is susceptible to failure by mechanisms other than those typically experienced with steel pipe. The losses and remediation costs associated with a simple pipe failure can be extraordinary, as a failed pipe can cause significant water damage to a high-rise building. Identifying the cause(s) of pipe failure can be challenging for a number of reasons. First, CPVC pipes can fail by several mechanisms, including high pressure, impact, environmental stress cracking (ESC), and manufacturing defects. ESC can be particularly vexing during a failure investigation. Potential ESC agents include many lower molecular weight and highly mobile compounds, which are soluble in the polymer. Common construction compounds such as adhesives, plumber's putty, fire barrier caulks, cutting oils, and glycol-based antifreezes are potential ESC agents. Second, the situation is complicated by the fact that only trace amounts of a contaminant may be necessary to cause ESC. The pipe failure event also tends to flush out the contaminants, further reducing their concentration on the pipe surface. What little contaminant remains can be difficult to identify or “fingerprint.” In addition, pipe defects including incomplete fusion, thermal damage, excessive residual stresses, or irregular molecular weight distributions may also contribute to ESC failures. Yet, they are not always apparent or easy to detect. This paper explores the challenges encountered during numerous failure investigations and reviews the state of the art in testing and analysis as applied to ESC in CPVC pipes.