This doctoral thesis analyses the impact of cohesion policy instruments on the regional development of the new Member States of the European Union. The research consisted of theoretical and empirical analysis. The theoretical part of the doctoral thesis includes absorption analysis and opportunities of new European Union states in the usage of financial instruments of cohesion policy. Analysing fund absorption from cohesion policy instruments, the new EU member states (EU13) are more slowly absorbing resources in comparison with the older EU member states (EU15). According to the literature, the foremost factor in successful adsorption is the institutional system of the Member State, following that the key to the successful usage of EU funds is strengthening of the human resources and supporting institutions. The empirical part of the doctoral thesis includes two-panel data analysis. The first analysis is directed to the determination of disbursement impact from ESI funds on the development of the NUTS 2 regions of the new Member States on GDP per capita. Indicators for 57 NUTS2 regions of the new Member State in the period from 2008. to 2016. were taken into the panel. After conducting statistical and dynamic panel analysis, it was concluded that disbursed funds, from ESI funds, had a small positive impact on GDP per capita. The second empirical analysis determines the impact of contracted funds, from ESI funds, on the development of NUTS 3 regions of the Republic of Croatia in the period from 2013. – 2017.. Based on the conducted statistical analysis of panel data, it was concluded that contracted funds have a small impact, but a positive impact on GDP per capita of NUTS 3 regions or counties in the Republic of Croatia. The most important lesson of this doctoral thesis is that it must not rely only on cohesion policy instruments as the only sources of investment and drivers of development. They can be used as a quality basis for the growth and development of the regions of the new Member States, but only cohesion policy instruments will not lead to regional convergence nor in Croatia or within European Union.