The doctoral dissertation covers the assessment of relative efficiency of wind power companies in electricity generation. The basic purpose of this doctoral dissertation is to identify the factors that will improve the efficiency of companies in the generation of electricity from wind energy and thus affect the efficiency of wind power plants as important producers of electricity from renewable energy sources. Therefore, the objective of the research is to explore and identify the causes of inefficiency of wind power companies and use the obtained knowledge to eliminate them, as well as provide information on the scope, i.e. size of possible improvement of certain factors of efficiency in electricity production from wind energy. It is important to note that absolute efficiency is not assessed in this doctoral dissertation, but rather the relative efficiency of wind power companies, and the efficiency of each entity is measured, compared and evaluated in relation to other entities in the reference set. In this respect, the Data Envelopment Analysis method was applied as the basic scientific research method (input-oriented DEA model), which is used to assess the level of efficiency of wind power companies in the generation of electricity at a specific point in time (one calendar year) – static conditions of the analysis. The research was based on random selection and covered and analysed 78 wind power companies from the Republic of Croatia and another 11 EU Member States. The sample includes only the companies that use wind energy in the production of electricity (and its transmission to the electricity network). This is the only activity of the analysed companies, and all other energy companies that generate electricity from different energy sources are therefore excluded from the sample. This supports one of the basic preconditions of the data envelopment analysis methodology, that the companies are mutually comparable and that they operate in similar conditions. In this doctoral dissertation, the factors (inputs of the model) used to assess the relative efficiency of wind power companies are classified into variables of technical and economic nature. By analysing average improvements of input variables in the model, it is possible to observe relatively equal amounts of projections with slight deviations in the values. The above is in line with the expectations, given the necessity for quality management of all aspects of company operations, i.e. total resources of the wind power company (economic and technical characteristics of wind power plants) in order to achieve balance between the involved production factors and optimal production levels in technical and economic terms. This supports the basic hypothesis of the doctoral dissertation according to which it is possible to extract the factors that affect the efficiency of wind power companies in the electricity generation, confirm the importance of management of total company resources, i.e. economic and technical characteristics of wind power plants, and use this knowledge to directly influence the efficiency of wind power companies in the Republic of Croatia, and consequently the efficiency of the electric power sector in the field of renewable energy sources.