The end-to-end evaluation of distributed systems' quality-of-service (QoS) properties such as performance, reliability, and security has historically occurred late in the software life cycle. As a result, many design flaws that affect QoS aren't found and fixed in a timely and cost-effective manner. Model-driven engineeringparticularly, domain-specific modeling languages (DSMLs) coupled with system execution modeling toolscan enable agile development of distributed systems and facilitate continuous system integration testing to improve quality assurance of QoS properties throughout the software life cycle. For example, the authors have realized such agile techniques in an open-source DSML-based system execution modeling tool called CUTS (Component Workload Emulator [Coworker] Utilization Test Suite). They've used CUTS as a case study to qualitatively and quantitatively evaluate how DSML-based system execution modeling tools can support lightweight and adaptable software development and QoS assurance processes. © 2006 IEEE.