This paper explores the conceptual framing and measurement of transboundary impacts in the context of the 2030 Agenda. It starts by defining transboundary impacts and reviewing different measurement approaches used so far. It then proposes a typology of transboundary impacts, classified depending on the type of international flows involved: financial flows, trade flows, movements of people, environmental flows and knowledge transfers. For each of these flows, transboundary impacts can be either positive or negative, depending on the aspect considered and on the conditions in origin and destination countries. Based on this framework, the paper presents evidence from a qualitative survey of experts about the potential impact of these five flows on each of the 17 Goals and 169 targets of the 2030 Agenda. Transboundary impacts are deemed by experts to be quite pervasive across SDGs, but also limited in scope to a small number of well-identified targets. Finally, the framework is operationalised for some specific areas within each of the five types of flows mentioned above, with the help of some proxy indicators. At the global level, the five types of transboundary relationships are dominated by three macro-regions, namely China, the United States-Canada and Europe, mainly reflecting the large size of these regions in most cases. When the assessment is conducted in relative terms (i.e. when impacts are normalised by population size or GDP), the picture becomes more nuanced, as 7 out of the 11 world regions considered record at least two large transboundary impacts. While this operationalisation is only meant to show how the proposed framework could be applied to concrete cases, the paper recommends its applications to other areas within each of the five flows, based on a richer set of indicators.