The Department of Defense (DoD) recently adopted evolutionary acquisition, a dynamic strategy for the development and acquisition of its defense systems. Evolutionary defense systems are planned, in advance, to be developed through several stages in a single procurement program. Each stage is planned to produce a viable system which could be fielded. The system requirements for each stage of development may be specified in advance of a given stage or may be decided at the outset of that stage's development. Due to the different stages that comprise an evolutionary system, there exists a need for careful reexamination of current testing and evaluation policies and processes, which were designed for single-stage developments. The Office of the Under Secretary of Defense for Acquisition, Technology and Logistics (USD-AT&L) and the Director of Operational Testing and Evaluation (DOT&E) asked the Committee on National Statistics (CNSTAT) of the National Academies to examine the key issues and implications for defense testing from the introduction of evolutionary acquisition.The CNSTAT was charged with planning and conducting a workshop to study test strategies for the evolutionary acquisition. The committee reviewed defense materials defining evolutionary acquisition and interviewed test officials from the three major test service agencies to understand the current approaches used in testing systems procured through evolutionary acquisition. The committee also examined possible alternatives to identify problems in implementation. At the workshop that took place on December 13-14, 2004, the committee tried to answer many questions including: What are the appropriate roles and objectives for testing in an evolutionary environment?, Can a systematic, disciplined process be developed for testing and evaluation in such a fluid and flexible environment?, and Is there adequate technical expertise within the acquisition community to fully exploit data gathered from previous stages to effectively combine information from various sources for test design and analysis?. Testing of Defense Systems in an Evolutionary Acquisition Environment provides the conclusions and recommendations of the CNSTAT following the workshop and its other investigations.