Research on domain-specific knowledge and general knowledge such as strategies has shown that information can indeed be available to a subject and still not be used. Several hypothesis have been set forth to explain this phenomenon; they as briefly exposed. An alternative, complementary hypothesis is then presented. It is assumed that most activities have several components organized in the form of a complex hierarchy. These interdependent components require monitoring because, at certain times, they are competing with one another for resources. Competition for resources and problems in component coordination may lead to a drop in performance. Several studies dealing with diverse knowledge domains are reviewed, which provide evidence for such resource problems. The same theoretical framework is then applied to explain several facts concerning teaching and learning: the impact of advance organizers; the instability of performance across repeated executions; some effects of social variables; the efficiency of tutoring and of taking into account the ‘proximal zone of development’. The limited cognitive capacity construct proves extremely useful in interpreting a large number of phenomena in a wide variety of domains.