Synthesizing a decade of research, the author describes a taxonomy of meaningful learning. The most meaningful learning outcome is problem solving. In this taxonomy, four different kinds of problem solving are arranged in a hierarchical manner. Prerequisite/corequisite with problem-solving outcomes are two fundamental reasoning skills that underpin all problem solving: analogical and causal reasoning. These forms of reasoning call on concepts-in-use, and lower- and higher-order propositions comprised of concepts and relationships. Problem-solving instruction should engage learners in solving problems, analogical comparisons of structurally similar problems, and analysis of the causal relationships (higher-order propositions) contained in the problem space. (Contains 1 figure.)