Present arrangements and regulation for ancillary services for power balance in power systems cannot cope with future developments in power systems as the participants do not receive proper incentives for required behaviour. This paper analyzes the consequences of current arrangements for system operation and stability when all participants make their own trade-off between risks and economic operation. Two-sided markets for ancillary services are proposed to replace the single-sided market of present secondary control arrangements and a price-based control strategy for power imbalances is described which can replace primary control. In contrast with present arrangements for primary control, all participants receive proper incentives such th...