The Threshold Project focuses on the language and learning difficulties that children in Southern African schools experience when they change from their mother tongue to English as a medium of instruction in their fifth year of schooling. This report discusses Threshold Project research relating to the nature of pupils' cognitive capacities and the ways in which pupils approach certain types of problem-solving tasks. Chapter 1 provides background on the Threshold Project and discussion of reasoning as an explanatory construct, social interest in reasoning skills, and the relation between culture and the curriculum. Chapter 2 provides an overview of theories of intellectual competence and their implications for curriculum development, focusing on: (1) Piagetian genetic epistemology; (2) information process systems approaches; (3) metacognition as a theoretical construct; (4) Pascual-Leone's theory of constructive operators; (5) cross-cultural cognitive psychology; (6) the neo-Piagetian model of Le Bonniec; and (7) the Vygotskian paradigm of cognitive development. Chapter 3 covers approaches to the teaching of thinking that concentrate on cognitive operations, an orientation towards heuristics, formal thinking, thinking through language and other systems, and thinking about thinking. Finally, Chapter 4 presents a model for the place of thinking in the curriculum and concludes with a summary of practical implications. (AC)