The article examines some of the issues surrounding program effectiveness; what is important to measure; what is known about program effectiveness; and what harm is done by the lack of effective programs. The article discusses measurement of outcome and the limitations of recidivism; measurement of program quality; principles of effective intervention; program implementation and leadership; offender assessment and classification; program characteristics; characteristics and practices of the staff; evaluation and quality control; treatment harm; and what can be done to evaluate correctional intervention programs. A search of the literature on program effectiveness disclosed that: (1) programs that meet the principles of effective intervention are more effective than programs that do not; (2) sound empirical research and periodic evaluation of correctional programs is critically important; and (3) evaluation efforts cannot afford to ignore the measurement of program integrity. Figures, notes, references