Interactive online diaries are a novel approach for evaluating project implementation and provide real-time communication between evaluation staff members and those implementing a program. Evaluation teams can guide the lines of inquiry and ensure that data are captured as implementation develops. When used together with conventional evaluation strategies, interactive online diaries allow for an in-depth understanding of project implementation, as well as the challenges program staff members confront and the solutions they develop. Interactive online diaries also can help evaluators address challenges such as self-reporting bias, documenting project evolution, and capturing implementers' ongoing insights as they develop. These insights might otherwise be lost to the evaluation process. The authors describe the development and use of this online approach in the evaluation of a foundation-sponsored program to improve the provision of preventive care in physicians' offices. The program included 17 practice-based research networks and their participating primary care practices. (Contains 1 figure and 3 tables.)