Forest inventory data, observational studies (mensurative and manipulative), and planted experiments provide the main data sources for the study of mixed forests. Demonstrative experiments such as marteloscopes are also basic to both research and training. Under the umbrella of the EuMIXFOR project, several research efforts have been conducted to further our knowledge as regards the functioning and development of mixed forests. Within this project, information from different forest research experiments at European level focusing on mixed forests has been compiled in order to identify the current state of the art. In the case of the European beech and Scots pine mixture, a gradient of triplets of monospecific and pure plots of these species has been established across Europe. Common establishment and measurement protocols have been developed in order to harmonize comparisons and analyses. Data sharing is also an objective; therefore, data from the triplet study of the European beech and Scots pine mixture are now available. A common dataset with the National Forest Inventory data from some European countries has been used for the study of mixtures.