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 (ID) 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 then 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 (“ontology”), theory-oriented ID (epistemology), and method-oriented ID (methodology). Different philosophical thought traditions can be related to these distinguishable meanings. I will show that the knowledge system mainly focuses on object-oriented ID. This type of ID is mostly present in what might be considered as “technological knowledge”. I will then clarify the notion of “problem” by looking at three systematic elements: an undesired (initial) state, a desired (goal) state, the obstacles and 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. – The objective of this paper is to foster and facilitate the theory discourse on interdisciplinarity, in particular a conceptual foundation of problem-oriented interdisciplinarity by finding a demarcation line between this type of interdisciplinarity and other types.