The present diploma thesis evaluates the question of conflicting land use interests in Austria in light of increasing land consumption and limited amounts of land as a natural resource. Hereby, the focus is especially laid on so-called public interests, which according to various international and national legal matters can serve as justifications for interventions into natural landscapes. The main questions of this thesis thus are: how is public interest defined, what are known possibilities to measure and evaluate these interests and what is the Austrian practice of weighting interests in the context of infrastructure projects? Starting with an overview of EU-directives and Austrian laws relevant for spatial planning, this thesis shows that the term public interest occurs in a variety of meanings whose differentiation is difficult. Furthermore, depending on the respective law the diverse interests are considered and weighted differently. In an additional step, existing methods of economic project evaluation are presented and discussed. Hereby, emphasis is placed on how to characterize the different approaches in terms of their evaluation procedure, the factors rated and the possibility to consider public interest. Methods that monetarily weigh environmental goods to allow a comparison with goods having a market value seem to be most suitable for comparison and weighting of different interests. Especially in the case of weighting decisions the decision-making process could be made more transparent through the use of multi-criteria-methods and a clear communication of the subjective criteria for weighing. Lastly, the theoretical knowledge gained is assessed for its practical application in two case studies: one located in Anif near Salzburg and one in Lech in Vorarlberg. In both cases the interest of natural conservation had to be weighed against other public interests such as land use planning, regional economy and tourism. However, the previously considered methods of economic evaluation were only partially used here. Instead, the methods employed were limited mostly to a verbal description of the project-affected interests. This circumstance is criticized in expert interviews that were conducted with participating environmental lawyers and experts on the projects. Accordingly, the present valuation of different interests is lacking precise limitations regarding the definition of public interest as well as guidelines for their assessment. The transparency of weighting decisions concerning public interest thus needs to be increased which might be achieved by the use of methods of economic evaluation.