Randomized experiments are an increasingly popular way to evaluate the impacts of development interventions. They provide hope that we can overcome important biases common to nearly all statistical evaluations. When done well, randomized control trials (RCTs) can provide clear, transparent, and credible evidence in complicated contexts, but their design and implementation is subject to a number of challenges. This note reviews some of the technical issues surrounding RCTs, describes four examples from microfinance, and discusses their advantages and drawbacks relative to other evaluation approaches.