The Transmission System Operator (TSO) is responsible for a stable and secure exploitation of the electricity grid in its control zone. This is mainly achieved by contracting power generators delivering ancillary services in addition to their main commercial product, active power. Electricity from Renewable Energy Sources (RES-e) is today generally exempted from the participation in ancillary services. However, the increasing share integration of variable RES-e with a limited predictability has an impact on the demand and supply structure of these services. In this paper, the possibility of wind power participating in frequency control or delivering active power reserves as an ancillary service is investigated. Within this framework, technical, regulatory and economic aspects are examined and evaluated. As the specific details about the way ancillary services are contracted differ over Europe, a case-study is done for the Belgian control zone.