Sometimes, program evaluations report no difference in outcomes between persons given a program and those not given the program. Is this because the program does not work, or because it was poorly implemented? Achieving high quality program implementation is critical to achieving anticipated outcomes, and researchers have made considerable progress in clarifying its importance in the past several decades. This brief defines program implementation, highlights the importance of high quality implementation, identifies key factors that affect implementation, presents the steps involved in achieving quality implementation, and specifies who has responsibility for quality implementation. The last section describes some practical lessons that have been learned about implementation through systematic research and practice. The focus here is on evidence-based programs, although implementation is relevant in all program operations and evaluations. Whenever any program is being conducted, it is important to monitor the level of implementation that has been achieved so its impact can be interpreted appropriately. "Resources That Provide Assistance on Selecting and Implementing Programs" are contained in the appendix. (Contains 2 tables and 1 figure.)