A logic of entailment is one in which it represents its own derivability relation as a connective. This allows it to express nested claims about what is derivable from what. It can say, for example, that if B is derivable from A and if C is derivable from B, then C is derivable from A. Mathematicians and logicians think this way when making proof plans. This chapter sets out the problem of constructing proof plans and looks at the other uses to which logics of entailment have been put. One key use, especially in the context of this book, is as a theory of the closure of scientific theories.