The paper presents a study of the impact of increasing amount of distributed generation (DG) on control and management (operation) of transmission and distribution systems. Paper considers how DG penetration influences frequency control during normal and emergency operational conditions. Controllability of different types of DG associated with frequency control is considered. List of power system parameters influencing frequency control is presented. Case study is based on analysis of frequency control aspects for the Baltic States power system interconnection. Methodology for analysis of DG penetration aspect is based on determination of hosting capacity – maximal level of possible DG penetration that provides efficient frequency control. Numerical results of case studies are presented in paper.