A design may be improvable without presenting an unacceptable risk related to safety or performance. However, design-related failures can result from an oversight in performing one of the major design activities or from a failure to balance the competing demands inherent to part design. This article focuses on design-related failures in products utilizing polymeric materials, and reviews important considerations of the design envelope of plastic parts. The article provides a non-exhaustive list and descriptions of design tools that can support the design process and the prevention of design-related failures. It also discusses the most common causes of design-related failures of plastic parts. The article can assist in both failure analysis and in the prevention of failures in which design may be a contributing factor or a root cause.