This book adopts an integrated history and philosophy of science approach to consider the historical origins and methodological pitfalls of field experiments in economics. It explains key concepts such as control and randomization and identifies two distinct origins of field experimentation in economics: controlled laboratory experiments and randomized field trials. Careful historical analysis reveals that two strands of field experiments have developed, with rather different methodological rationales regarding the validity, both internal and external, of inferences about experimental results. The authors propose a unified methodological framework that makes explicit the complementarity between the two strands of field experiments, as well as between experimentation and other research methods such as econometrics.