The insurance industry has a very important role in starting the overall economy not only in the Republic of Croatia but also much wider. That is why the insurance industry and determining its efficiency are becoming a topic that an increasing number of authors are dealing with. This thesis seeks to determine the efficiency of non-life and complex insurance companies in the Republic of Croatia on the basis of data from the profit and loss account in the period from 2013 to 2019 in order to see whether the introduction and application of the Solvency II Directive brings the most significant changes in the operations of insurance companies and reinsurance companies had an impact on their efficiency. To determine the efficiency, the method of Data Envelopment Analysis was used, which is defined as a method used to determine the performance of insurance companies because it is very suitable in cases where the efficiency of a large number of decision makers needs to be determined. Data Envelopment Analysis is a method by which each decision maker is compared to the best, and the goal is to find the best virtual decision maker for each real unit. The aim of this thesis is to determine the efficiency and see how the introduction of new legal and regulatory provisions of the Solvency II directive has affected the change in the efficiency of the observed decision makers. After the Data Envelopment Analysis using the model focused on inputs, which was performed in software package "deaR" and the programming language "R", the results were obtained that show about the efficiency of the observed decision makers. From the total of fourteen non-life and complex insurance companies, four of them achieved the maximum efficiency they have had since 2016 when the Directive was introduced by the end of the observed period (2019), six companies were gradually improving their business results and efficiency, and only two companies experienced a decline in efficiency after the introduction of the Solvency II Directive. In conclusion, the introduction and application of the rules of the Solvency II Directive had a positive effect on the efficiency of the observed insurance companies in the observed time.