The aim of this chapter is to provide the basic principles, examples and possible causal models of diachronic changes that lead to the emergence of efficient language structures. I argue that they are at least partly motivated by the pragmatic mechanisms and heuristics discussed in the previous chapters. This chapter provides diachronic evidence of efficient reduction and enhancement. Alternative explanations are also discussed. I offer an overview of influential theories that specify the causal mechanisms of language change leading to efficient formal and semantic changes, including grammaticalization. In addition, the chapter addresses the motivations for suppletive and compositional forms, emergence of efficient word order patterns and a more philosophical question about the role of teleology in explanations of language change.