This chapter addresses the history of the English system of clausal complementation. It is organised around four major questions. First, where do complement clauses (or CCs) come from? The history of English indicates that adverbial clauses can turn into CCs (e.g. lest-complements), or phrasal units undergo clausalisation (e.g. the gerund) – or both these mechanisms come into play (e.g. the to-infinitive). Second, what changes can CCs undergo? Changes to CCs may affect their internal syntax. For example, subjectless non-finite clauses have a strong tendency to develop subjects (e.g. ECM constructions, for…to-infinitives, secondary predicates). Often, CCs also undergo distributional change as they spread to new CC-taking predicates. The characteristic pattern is one of lexical diffusion. Third, how does the system change as a whole? English sees an unmistakable trend towards more non-finite complementation – a development known as the ‘Great Complement Shift’. This leads to a great number of variation hotspots, where finite CCs compete with non-finite alternatives, or non-finite alternatives compete among themselves. Fourth, what eventually becomes of CCs? At least two pathways of change appear to be open to CCs. In both cases CCs become more main-clause like. Either the matrix clause develops into an operator (i.e. an auxiliary or parenthetical), or the matrix clause disappears altogether, leading to insubordination.