In the Swiss National Forest Inventory (NFI), data on about 6400 sample plots is collected directly in the field, but not all information of interest can be assessed in this way. During NFI1, an interview survey was therefore designed to complement the terrestrial inventory. All local Swiss foresters (about 800) responsible for the forest districts and representatives of the forest authorities (forest service) in Switzerland were interviewed. The interviews were conducted twice in NFI4: the first for the first five NFI annual panels and the second for the remaining four annual panels (Sect. 8.3). This procedure resulted in one interview for each sample plot for every NFI cycle. The interview survey consists of two main parts: interviews to obtain information about the assessed sample plots, conducted twice during NFI4, followed by interviews conducted only once about forest roads throughout the entire country (independent of the sample plots). The interviews in NFI4 covered about 50 different attributes.