This book contains a systematic analysis of the notion of work ability. The analysis is particularly focused on the following three contexts: medical insurance, training for a vocation or a profession and evaluation of a job seeker. The first part of the book presents and briefly analyzes some methods and instruments designed for characterizing and assessing work ability. The second part gives a theoretical foundation for the analysis of work ability. Here the author presents an original taxonomy of competencies and other qualifications for work and proposes a new definition of work ability. The third part is devoted to the issue of work ability and its relation to medical conditions. In addition to a description of certain legislations in the medico-insurance field, this part of the book contains a substantial analysis of the notion of disease and an attempt to determine whether there is a viable notion of disease that can answer the demands of objectivity raised by offices of social insurance.