Achievement gaps concern educators at all levels today. Educators recognize the threats these gaps pose to education quality and equity, and they are working hard to close them--but an equally threatening gap in education with consequences just as serious is largely ignored. It influences every educational-improvement effort and seriously jeopardizes the chances of success. That gap is the one between the educators' beliefs about the characteristics of effective professional development and the evidence they have to validate those beliefs. Having acknowledged the gap between beliefs about effective professional development and the scientifically valid evidence that supports these beliefs, what can educators do to reduce that gap? In this article, the author discusses ways on how to close the knowledge gap on effective professional development. The first requirement is honesty about what trustworthy research on professional development has actually revealed. Second, educators must begin demanding better evidence from consultants who claim their ideas, strategies, and practices are "research-based." Third, they need to get serious about evaluating all forms of professional development. Fourth, they need to press researchers to study professional development more rigorously. Finally, they need to be honest about the real world of schools and the powerful influence of context.