To use environmental program evaluation to increase effectiveness, predictive power, and resource allocation efficiency, evaluators need good data. Data require sufficient credibility in terms of fitness for purpose and quality to develop the necessary evidence base. The authors examine elements of data credibility using experience from critical appraisal of studies on environmental interventions employing systematic review methodology. They argue that critical appraisal of methodological quality is a key skill to improve both retrospective evaluation and prospective planning of monitoring of environmental programs. Greater transparency and data sharing among evaluators could facilitate rapid development in approaches to environmental evaluation that improve data credibility.