This chapter explains how to articulate a clear study purpose and then choose a complementary nonexperimental or experimental study design. A flowchart aids design selection, while figures and boxes illustrate many details. Nonexperimental descriptive, correlation, cohort, and case-controlled designs are described, as are experimental randomized controlled trials, quasi-experiments, and posttest only, cross-over, and factorial designs. Other less common designs–mixed methods, methodological, ideological, and historical–are also briefly explained. Finally, the dynamic, cyclical research-theory relationship is summarized.