This paper presents a theorisation of collaborative activity that was developed in the research field known as “collaborative learning”, in order to understand the processes of co-elaboration of meaning and knowledge. Collaboration, as distinguished from cooperation, coordination and collective activity, is defined as a continued and conjoined effort towards elaborating a “joint problem space” of shared representations of the problem to be solved. An approach to analysing the processes of co-construction of a joint problem space is outlined, in terms of inter-discursive operations, together with approaches to defining different forms of cooperative activity. In conclusion, the specificity of this approach to defining collaboration is discussed in relation to other fields of research.