This paper focuses on the relationships between observed patterns of innovative activities within a sector and the related context and underlying microeconomic processes that might account fur them. it claims that there are some invariant features (with respect to relative prices and incentives mechanisms) of learning and knowledge accumulation that greatly affect the rare and structure of innovative activity. These features are different across sectors. The paper proposes that the specific pattern of innovative activity of a sector can be explained as the outcome of different technological regimes that are implied by the nature of technology and knowledge. The notion of technological regime provides a synthetic representation of some of the most important economic properties of technologies and of the characteristics of the learning processes that are involved in innovative activities.