The evaluation literature often debates whether evaluators should be flexible in evaluation design and activities in order to collaborate with program directors and be responsive to programming needs. Two conditions are specified under which evaluation flexibility is not only desirable but essential. Two examples from the cluster evaluation of the W. K. Kellogg Foundation's Community Partnerships for Health Professions Education initiative are provided to illustrate why flexibility under these conditions proved to be essential. One of the examples, related to the "community" involvement in the initiative, illustrates the need for flexibility as programs experience goals clarification. The other example, related to the coincidental national health care reform efforts, illustrates the need for flexibility both to capture programs' efforts to protect their integrity and to ensure against spurious conclusions as a result of external turbulence in policy environments. How the cluster evaluation team addressed these issues is also described.