Deductive reasoning is seen as having two components. One, the logical component, consists of a logical vocabulary for composing propositions, and a repertory of deductive steps. The other, the performance component, includes two main programs, (1) comprehension processes that determine the information accepted from the premises, and (2) routines and strategies for constructing a line of reasoning. The research has two objectives: to define the logical component, and to clarify the nature of the improvement in deductive reasoning that takes place from about age ten to adulthood. The work on the logical component starts from a conception that the logical function of if-then is to provide a grammatical frame for stating inference rules; this view is incorporated into a set of inference rule schemes that claim to define the repertory of deductive steps used in propositional reasoning. The model covers the domain covered by standard propositional logic. The work aims to test this model, and to extend it to reasoning from quantified premises. In the main method used, the model, considered in conjunction with plausible performance assumptions, is made to predict the mental steps that people go through in solving problems: differences in errors and/or response times as a function of the kind of problem are predicted and tested in experiments with adult subjects. Introspective protocols are also gathered on subjects' intermediate steps in solving problems, from which their inference rules can often be inferred. The developmental changes are expected to lie in the performance component. An hypothesized change with age in comprehension processes is used to predict a pattern of variation in errors and response time across kinds of syllogistic reasoning problems for children, and to predict how the error pattern should change as the content of the premises is made more explicit. In another study, hypothesized child-adult differences in heuristics for constructing an argument lead to predictons about the reasoning problems children will make errors on.