Boundaries are ubiquitous across a wide range of ecological systems and spatial scales. However, most research on boundaries has been scale and system specific. To promote the synthesis of boundary studies across the range of environments and scales they represent, we present an inclusive scope for boundary studies. Three linked tools make the scope operative: (1) a causal framework covering all types of boundaries, (2) a model template, and (3) a strategy for constructing hypothetical models of boundary function in any ecological system. The framework focuses on flows of organisms, materials, energy, or information in heterogenous mosaics; it specifies patch contrast, identity of the flow, and nature of the boundary as the concepts to quantify in any model. The model template arranges these components in a functional form to elucidate specific boundary relationships. From the model template, working models that are system and scale specific can be developed. We exemplify the use of the linked tools of framework, model template, and working model with an experimental study of forest–field boundary function.