1 This is a post-print version of an article published in European Planning Studies in 2014. There may be minor differences between this version and the printed version. Please refer to the journal article: Steen, M. & Hansen, G.H. 2014. Same Sea, Different Ponds: Cross-Sectorial Knowledge Spillovers in the North Sea. European Planning Studies, Vol. 22, 2030-2049. DOI: 10.1080/09654313.2013.814622 Same Sea, Different Ponds: Cross-Sectorial Knowledge Spillovers in the North Sea MARKUS STEEN* & GARD HOPSDAL HANSEN** *Department of Geography, Norwegian University of Science and Technology, Trondheim, Norway. ** Centre for Sustainable Energy Studies (CENSES), Norwegian University of Science and Technology, Trondheim, Norway. Correspondence Address: Markus Steen, Department of Geography, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Email: markus.steen@svt.ntnu.no Keywords: Knowledge spillovers, relatedness, oil and gas, offshore wind ABSTRACT Knowledge spillovers are crucial to innovation and upgrading, but it is largely unclear what knowledge spillovers are made of and how they actually happen. The importance of MAR vs. Jacobs externalities is also a debated matter, whereas the concept of "related variety" has recently come to occupy a middle-ground position. However, the relatedness concept is ambiguous in terms of operationalization and emphasises codified knowledge on behalf of other knowledge resources that are important for innovation, particularly if firms cross into new sectors. This paper sheds light on the "black box" concepts of knowledge spillovers and relatedness by exploring cross-sectorial transfers from the mature offshore oil and gas sector into the emerging offshore wind industry. A qualitative research design allows for a more nuanced understanding of the contents of knowledge spillovers and (un)relatedness between sectors.