This paper considers the efficient evaluation of recursive queries expressed using Horn Clauses. We define sideways information passing formally and show how a query evaluation algorithm may be defined in terms of sideways information passing and control. We then consider a class of information passing strategies which suffices to describe most query evaluation algorithms in the database literature, and show that these strategies may always be implemented by rewriting a given program and evaluating the rewritten program bottom-up. We describe in detail several algorithms for rewriting a program. These algorithms generalize the Counting and Magic Sets algorithms to work with arbitrary programs. Safety and optimality of the algorithms are also considered.