This paper examines the nature and significance of learning by experience during work, both paid and unpaid. Data about the relationship between costs, especially labour costs, and output have come to be interpreted as evidence of learning by experience, but these grouped data are unable to explain the nature and process of individual experience or learning. Direct and detailed evidence from recent case studies, analysed using theories of the open employment contract and continuous cognitive reformulation, illustrates an active, reiterative relationship between work and learning that generates productivity. The analysis of individual behaviour modifies concepts and theories of human capital and mobility, and also has wider implications for economic and social theories, and for research. (Contains 6 notes and 2 figures.)