Several criteria can be used to define industry, including the type of goods produced, the massification and standardisation of production, the specialisation of the work space, the segmentation and mechanisation of tasks, the proletarianisation of workers, or the target market, final destination of the production. It is this last criterion that has allowed historians to identify the industries that developed during the ancient and medieval periods. The addition of services in the classical economist definition of industry -which referred to activities that combine factors of production to produce material goods for an external market-shows that the notion of industry and its definition are in constant renewal. Historians are now interested in the multiple paths taken by industry over the centuries, going beyond a linear and evolutionary vision of modes of production and techniques: small units, home-based work and vast networks of subcontracting rub shoulders and sometimes combine with concentrated workshops and mechanised production lines. In order to go beyond a definition based on a series of debatable criteria or on a strict opposition to the notion of craft industry, the authors propose to define industry as what happens to a technical phenomenon when its quantitative growth leads to its qualitative transformation.