In the electric power system, electricity supply needs to be balanced with electricity demand and network losses at all times to maintain safe, dependable and stable system operation. Flexibility within the power system is required to compensate for variability in supply and demand and maintain its balance. Historically, variability and uncertainty in the system mainly occurred on the demand side. This variability was to be matched top down through flexible conventional power plants and a reliable grid. The ongoing changes in energy production and consumption – creating more variability and uncertainty – ask for other means of flexibility in addition to the conventional ones. The main changes are the increase of renewable energy sources, both on large-scale and distributed level, and the increase in power consumption and demand variability by the electrification of the transportation and heating & cooling sectors. The latter also provide a means of flexibility, that is demand side management. This means that new power consumers, like electric vehicle chargers and electrical heat pumps, can respond to flexibility requests from the power system by adapting their consumption pattern. This new ecosystem creates the smart grid, where renewable energy sources, flexible consumers and energy storage systems cooperate. The different sources of flexibility may have technological and cost-related barriers to prevent their uptake. There are also barriers in regulation, standardisation and energy market rules. However, regulations, standards and markets are being developed worldwide to facilitate emerging flexibility solutions, like energy storage and demand response. One of the standardisation initiatives is DNV GL’s recommended practice on energy storage, GRIDSTOR. For most flexibility resources, especially energy storage, a single flexibility service will not ensure a positive business case. Stackable revenues, by combining operational services in different markets and time scales, are therefore important to render the business case worthwhile. At the seminar I will explain the need for flexibility in the current and future power system, and present the types of services for flexibility, the available sources, the existing barriers for deployment, and a few simple examples for the business case. Here, I will mainly focus on battery energy storage systems.