Evaluators are becoming increasingly aware that, to provide maximum benefit to the programs they evaluate, they must address the systems within which the programs work. However, evaluation practice is often influenced by beliefs and assumptions that are rooted in an understanding of these systems as stable and predictable. We identify four traps that can limit the usefulness of program evaluations when the instability and uncertainty of systems are not acknowledged. We explore how evaluators can get "untrapped" by using a complex systems lens to interrogate their beliefs and assumptions about (1) the purpose of the evaluation, (2) the definition of the evaluand and boundaries of the evaluation, (3) leveraging controlled evaluation methodologies to learn about complex systems, and (4) the meaning of "going to scale." From our work on a national cross-site evaluation, we provide examples of ways a complex systems lens can increase the merit, worth, and significance of program evaluations.