Among others, the term problem plays a major role in the various attempts to characterize interdisciplinarity or transdisciplinarity, as used synonymously in this paper. Interdisciplinarity is regarded as problem solving among science, technology and society and as problem orientation beyond disciplinary constraints. The point of departure of this paper is that the discourse and practice of ID have problems with the problem. The objective here is to shed some light on the vague notion of problem in order to advocate a (...) specific type of interdisciplinarity: problem-oriented interdisciplinarity. The outline is as follows: Taking an ex negativo approach, I will show what problem-oriented ID does not mean. Using references to well-established distinctions in philosophy of science, I will show three other types of ID that should not be placed under the umbrella term problem-oriented ID : object-oriented ID, theory-oriented ID, and method-oriented ID. Different philosophical thought traditions can be related to these distinguishable meanings. I will then clarify the notion of problem by looking at three systematic elements: an undesired state, a desired state, and the barriers in getting from the one to the other. These three elements include three related kinds of knowledge: systems, target, and transformation knowledge. This paper elaborates further methodological and epistemological elements of problem-oriented ID. It concludes by stressing that problem-oriented ID is the most needed as well as the most challenging type of ID. (shrink)