The design and development of telecommunications software products require continuous attention to issues of performance and capacity, both of which are important requirements for the products that Lucent produces. The Software Technology Center (STC) in Bell Labs Advanced Technologies has defined performance-engineering best practices that help a project meet the performance and capacity requirements for its product. In this paper we describe these practices, which have been employed in Lucent for many years and on numerous projects. We also provide a detailed description of the role of the performance engineer, which is essential to these best practices. © 2003 Lucent Technologies Inc.