Hydroelectric plants, constructed as modern, industrial, innovative and technological structures of the 20th century, after approximately a century of existence, have become subjects of industrial heritage. Within the article, the interdisciplinary field is approached through the perspective of architectural conservation with consultancies of experts from related disciplines. The study discusses hydroelectric facilities of the past century in terms of industrial heritage focusing on their features, types and elements, investigates the theoretical framework in order to specify criteria for their assessment as cultural heritage, and develops a guideline for the architectural preservation, conservation, restoration and re-use of these structures. The proposed set of criteria and the guideline are applied for 17 selected case studies of dams and powerhouses in Northern Italy. The process and the results of the study are discussed with a purpose of serving as a model for further studies on the preservation, conservation and restoration of hydroelectric plants.