This paper reviews the energy production potential of historic hydro sites (watermills, weirs and other) located in 21 EU countries which so far have not been used for power generation. It is based on the data collected by the RESTOR Hydro project, consisting of some 65,000 hydro site data. The lion's share of this data is attributed to micro-hydro sites, mini and small hydro sites being left in a marginal position. The main aim of this paper is to explore the collected project data of micro-hydro sites in terms of scientific analysis, to present them in a consolidated form, to identify the countries with the largest hydro potential and most prominently to indicate those of them that fall into environmentally sensitive areas. It provides only representative data, evaluates the potential capacity and hydropower generation at sites of the individual countries, gives an insight on their restorability, exhibits the distribution of micro hydro sites within the EU network of protected areas (Natura 2000) and visualizes micro-hydro data all over the EU in a concise way. An approximate share of the generating energy resulting from the historic hydro sites in the remaining small hydropower potential of the investigated countries is identified (6.8 TWh/yr). (C) 2018 Elsevier Ltd. All rights reserved.