Planning and designing a program's evaluation during the program's initial design phase and prior to its implementation are essential for ensuring that the evaluation is scientific, meaning that the program's objectives and implementation are measurable and that appropriate data are collected for determining whether program implementation and outcomes have been achieved. Four guidelines are suggested for planning an evaluation before starting a program. First, choose the questions to be answered by the evaluation, and determine the information needed to answer them. Second, determine the timing of evaluations and the resources needed to conduct them. Third, document the program's design, purpose, and objectives so an evaluator can compare them to the program's actual implementation and outcomes. Fourth, remain flexible, so the evaluation plan can be revised as necessary. The article also discusses special evaluation challenges associated with multi-site programs. In addition, it addresses "evaluability assessments," which gauge how well the evaluation plan was conducted and its effectiveness in preparing the program for an evaluation.