Field experiments represent a relatively new area in economics to understand the causal links from one variable to another. They have been used by academics to help answer interesting and policy-relevant questions in the developed world relating to educational attainment, tax avoidance, consumer finance, negative externalities, charitable giving, and labour market contracts. In this paper we bring together the key ideas behind the different variants of field experiments, how field experiments have been used to test theory, their limitations, and the new areas currently being opened up by field experiments.