This article examines conceptualisations of scale in catchment science and regional studies as the basis for the development of an integrated characterisation of spatial units. Our starting point is the apparent similarity in the scalar terminology utilised in the two fields, pointing to an area of potential conceptual overlap between physical and human geography. While our aim is not to produce an overarching theory of scale, we seek to advance understanding within both catchment science and regional studies by comparing their approaches to the analysis and understanding of spatial units. We also describe underlying principles of heterogeneity and complexity in the functioning and behaviour of catchments and regions. After clarifying scale definitions, this article examines notions of ‘scaling’ and ‘rescaling’ in each field and assesses catchments and regions as complex and open systems. In the penultimate section of this article, we develop an analytical framework based on the identification of certain dynamic attributes and concepts of integration, drawing upon the meta-theoretical language of complexity science.