Old-growth forests and stands have multiple values, such as biological, scientific, socioeconomic, aesthetic, spiritual and cultural ones, but they are very rare in central Europe. Currently, an overview about the existence and size of old-growth forest patches in Switzerland is missing. We propose a definition for old-growth forests worth preserving based on ten criteria: 1) presence of veteran trees, 2) significant biomass, 3) heterogeneous stand structure including senescent and decaying stages, 4) presence of natural processes and disturbances, 5) high dead wood volume (at least 30 m3/ha) and a large diversity of dimensions and decay stages of dead wood pieces, 6) autochthonous tree species, 7) diverse and natural species assemblages, 8) only extensive or no harvesting for the last 50 years, 9) long continuity of forest cover, 10) minimal area of 30 ha for old-growth forests and 1 ha for old-growth stands. When searching for old-growth forests and stands worth preserving, quantitative criteria from the National Forest Inventory and other sources should also be used. We call on all forest specialists to identify old-growth patches, to inform the population and decision makers about their high value, and to protect them in an adequate manner as a unique natural heritage.