Evolutionary theory has found its way into a staggeringly large diversity of fields outside the biological sciences. This Element examines how crossovers of evolutionary theory from biology into other fields occur, and in what ways such fields can be meaningfully considered evolutionary fields of research. Cases of crossover of evolutionary theory have so far not been examined systematically by philosophers of science, and this Element aims to start with developing a philosophical account of this practice as a general strategy in science. It shows that theory crossovers do not consist in straightforward applications of a generally accepted version of evolutionary theory to non-biological phenomena, but must be understood differently. Alternatively, it is suggested that crossovers of evolutionary theory involve a general style of thinking and shown how this provides a unifying perspective on crossovers of evolutionary theory between fields. This title is also available as Open Access on Cambridge Core.