The theory of instrumental genesis of Rabardel relates the social and the technical through the concept of instrument. An instrument is defined as a mixed entity made up by an artifact, the technical/material part, and a set of utilization schemes, the social/behavioural part, which both result from users' constructive activities. This theory is not dedicated to learning contexts, but it can help illuminate many aspects of instrument-mediated collaborative learning situations and CSCL systems. In the first part of this article, the foundational concepts of Rabardel's theory are summarized and discussed. Drawing from that perspective, the second part of the article stresses (1) the complexity of CSCL instrument geneses mainly due to their dual nature--with both teachers and learners involved in the process, and (2) the multifaceted mediating role CSCL systems can play during both task performance and resources elaboration activities. It is argued that the relative importance of teachers and learners during instrumental geneses is the essential discriminating characteristic of CSCL systems. In the resulting categories ("user-instrumentalizable systems" and "teacher-instrumentalizable systems"), the degree to which systems support the constructive activities related to their own development is considered another important differentiating factor. The third part of the article aims at elaborating and illustrating with representative examples of CSCL systems that theory-based classification. The article concludes by suggesting a number of directions for further research in the field.