River connectivity is essential for the correct functioning of freshwater ecosystems at all scales. However, it has not received the necessary attention by researchers, managers and policymakers until recent years. In this review, we recap the state of knowledge in river connectivity and its applications to conservation. We describe the particular characteristics of river connectivity and summarise the effects of its interruption in different freshwater ecosystem elements. We then focus on the effects of the lack of segment connectivity in fish species and review the different methods developed to study it. The application of connectivity in freshwater fish conservation areas is also reviewed, which highlights the lack of studies on this subject. Finally, connectivity restoration is studied. The review addresses these topics in a general way and then focus on the Iberian Peninsula.The Iberian Peninsula is an interesting place to study river connectivity because it has one of the highest numbers of dams per square kilometre and a large number of endemic and endangered freshwater fish species. Despite the high number of fish species affected by water extraction and damming, river connectivity and its effect in Iberian freshwater fish populations have not been well studied. A small number of studies analyse the effect of small dams in nearby fish communities, but large-scale impact assessments are scarce.