In this report, flexibility is discussed in terms of the ability of a unit or a system of units to change their electricity production or consumption to participate in these ancillary services. In district heating units, the electrical flexibility is strongly correlated with the thermal flexibility which in turn is very site specific. This report makes some proposals on what measures could be carried out to improve flexibility and grade them by implementation difficulty and cost estimates. Most of the measures refer to some upgrades in CHP plants to increase these plants’ flexibility potential towards the ancillary servics. These measures affect not only the electricity output of CHP plants but also the heat output. The report also discusses several means to access heat supply flexibility, to support the CHP plants’ power flexibility. This report also serves as an easy introduction to heat production in district heating systems and, in particular to CHP plants, targeted at experts in other fields to get a basic understanding of some technical aspects and limitations of district heating systems.