The provision of control reserve, and therefore contributing to the secure operation of the power system, is paramount in a future energy system with increasing shares of fluctuating renewable energy sources. This doctoral thesis proves that fluctuating renewable energy sources, such as onshore and offshore wind farms as well as photovoltaic systems, are capable of providing control reserve at the same level of reliability as conventional generators. It is shown that the introduction of fluctuating renewables to the control reserve market can access a welfare gain that could be realized as additional income by the new market participants or as cost saving potential of the control reserve procurement. The dependency analysis between the welfare gain and the regulatory framework leads to recommendations for the development of the control reserve market.