This paper is a practical guide (a toolkit) for researchers, students and practitioners wish- ing to introduce randomization as part of a research design in the field. It first covers the rationale for the use of randomization, as a solution to selection bias and a partial solution to publication biases. Second, it discusses various ways in which randomization can be prac- tically introduced in a field settings. Third, it discusses designs issues such as sample size requirements, stratification, level of randomization and data collection methods. Fourth, it discusses how to analyze data from randomized evaluations when there are departures from the basic framework. It reviews in particular how to handle imperfect compliance and ex- ternalities. Finally, it discusses some of the issues involved in drawing general conclusions from randomized evaluations, including the necessary use of theory as a guide when design- ing evaluations and interpreting results. JEL Classification: I0; J0; O0; C93. Keywords: Randomized evaluations; Experiments; Development; Program evaluation.