Background: Observational methodology is one of the most suitable research designs for evaluating fidelity of implementation, especially in complex interventions. However, the conduct and reporting of observational studies is hampered by the absence of specific guidelines, such as those that exist for other evaluation designs. This lack of specific guidance poses a threat to the quality and transparency of these studies and also constitutes a considerable publication hurdle. The aim of this study thus was to draw up a set of proposed guidelines for reporting evaluations based on observational methodology. Method: The guidelines were developed by triangulating three sources of information: observational studies performed in different fields by experts in observational methodology, reporting guidelines for general studies and studies with similar designs to observational studies, and proposals fro experts in observational methodology at scientific meetings. Results: We produced a list of guidelines grouped into three domains: intervention and expected outcomes, methods, and results. Conclusions: The result is a useful, carefully crafted set of simple guidelines for conducting and reporting observational studies in the field of program evaluation