This chapter presents a brief theoretical overview of intelligence, cognition, and expertise and their theoretical basis for use in the subsequent chapters. It introduces the main models of intelligence including trait and factorial models, the triarchic mind, and multiple intelligence theories. It then reviews the approach of cognitive psychology based upon early computer modelling of human cognition, schemas and frames, production systems, and episodic and semantic memory. Finally, it reviews expert systems, expert knowledge acquisition and retrieval, practice, transfer of skill, flexibility of knowledge retrieval, and how all of these factors influence the ability of an individual to make transitions in their careers.