This paper elaborates on the question of how to design an epistemological foundation for problem-oriented, collaborative forms of research, such as transdisciplinary sustainability research. It picks up approaches of twentieth-century European philosophy to the concept of the problematic and design research. The problematic is explained as a historical epistemological effort. Design research shows parallels to the epistemological thinking of the problematic by contributing to a differentiation and historicity of knowledge and knowledge production itself. Designing is constituted by a nexus of conceptual thinking and creative making, and so designs are drafts themselves. We interweave the thinking of the problematic with the practice of designing in order to open an epistemological perspective in and for transdisciplinary sustainability research. We call this a ‘thinking practice of problematic designing,’ which describes an epistemological tool as well as a transformative process. Problematic designing is characterized by always being in the making–its designs can grow beyond their conditions of production. By opening up manifold dimensions of transformation, this epistemological approach is oriented towards complexity, enabling the generation of sound and future-relevant knowledge.