This article examines the implications for the EU's energy system if an 80% reduction in CO 2 emissions is to be achieved by 2050 against 1990 levels, using the European TIMES Model (ETM-UCL) to project a least-cost pathway that meets this CO 2 constraint ('Policy Success'), along with milestone targets for 2020. A Reference scenario (no CO 2 constraints post-2020) was analysed to allow for comparison. The key conclusions are as follows: (a) the achievement of negative emissions in the power sector via the use of biomass with carbon capture and storage (BECCS) allows for much more limited decarbonization in the buildings and transport sectors; (b) CCS is also extensively used for decarbonization of the industrial sector; (c) because of the absence in the model of options for transport mode-switching and building fabric efficiency improvements, the transport and buildings sectors achieve relatively little abatement by 2050 - the inclusion of these options could considerably reduce the need for BECCS and the cost of abatement; (d) decarbonization of the EU's energy system by 2050 would increase energy system costs by 14% compared to a Reference scenario with no CO 2 constraints; and (e) average EU-wide marginal CO 2 abatement costs in Policy Success reach $300/tCO 2 in 2050. Such a value is within the (wide) range of marginal carbon prices produced by comparable scenarios in other studies. Policy relevance The EU has set itself a target of reducing its greenhouse gas emissions by at least 80% by 2050, against 1990 levels. This will require a reduction of at least 80% in CO 2 emissions from the energy system. This article, using results from the European TIMES Model (ETM-UCL), demonstrates that in the absence of significant decarbonization in the buildings and road transport sectors, substantial negative emissions in the power sector must be achieved, through as-yet unproven technologies (involving BECCS) . Therefore, a comprehensive strategy to reduce emissions across all energy-using sectors is required if this outcome is to be avoided. In addition, this article adds further evidence to the notion that substantial decarbonization of the EU's energy system may be achieved through a relatively small additional investment above that required anyway.