A natural language processor is described for control of a machine in task-oriented situations. Particular emphasis is given to issues related to flow-of-control statements in dialogue. These include branching constructs, as in `if row 1 contains a positive entry, then . . .' and looping constructs, as in `repeat for all other rows'. Special problems are discussed concerning the processing of deeply nested control structures, pronoun resolution, and the handling of conjunctions. An experiment is described in which the robustness of the conditional feature was tested with a group of computer naive subjects. It was found that subjects could discover and use the feature effectively in solving problems even though the fact of its existence was systematically withheld during the training session.