The study of groundwater–surface water interaction has attracted growing interest among researchers in recent years due to its wide range of implications from the perspectives of water management, ecology and contamination. Many of the studies shed light on conditions on a local scale only, without exploring a regional angle. To provide a broad and historical overview of groundwater–surface water interaction, a review of research carried out in Denmark was undertaken due to the high density of studies conducted in the country. The extent to which this topic has been investigated is related to Denmark's physiography and climate, the presence of numerous streams and lakes combined with shallow groundwater, and historical, funding, and administrative decisions. Study topics comprise groundwater detection techniques, numerical modeling, and contaminant issues including nutrients, ranging from point studies all the way to studies at national scale. The increase in studies in recent decades corresponds with the need to maintain the good status of groundwater-dependent ecosystems and protect groundwater resources. This review of three decades of research revealed that problems such as the difference in scales between numerical models and field observations, interdisciplinary research integrating hydrological and biological methods, and the effect of local processes in regional systems remain persistent challenges. Technical progress in the use of unmanned aerial vehicles, distributed temperature sensing, and new cost-effective methods for detecting groundwater discharge as well as the increasing computing capacity of numerical models emerge as opportunities for dealing with complex natural systems that are subject to modifications in future triggered by climate change. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water and Environmental Change Water and Life > Nature of Freshwater Ecosystems.