Energy industry players and policy makers must make many long-term decisions that may have an impact on the energy system for many years to come. These include decisions on energy technology investments, taxation and subsidy schemes and public infrastructure such as power transmission lines. Strong modelling tools that accurately represent the energy system are required in order to quantify the costs and benefits of each decision in advance, in order to determine the correct decision. In the past, energy decisions were often taken on the basis of results from isolated models, for example, a model of the electricity system that did not incorporate the gas system, or a model of home heating that did not include the water sector. MSP1’s primary aim is to develop integrated models that consider multiple energy systems in an integrated manner rather than focusing on individual sectors in isolation. These models will be used to quantify benefits of energy systems integration across pathways and scales. MSP1 will link in with many other work packages by using outputs from those work packages as inputs for the models. For example, MSP1 will use modelling of home heating demand from EUI3 to model the electricity generation system and the electricity transmission system in an integrated manner, and thereby determine the impact of different heating demands on both generation and transmission requirements. Another example would be the use of insights from EUI4 on energy demand and its flexibilities in wastewater treatment to model and analyse potential benefits of harnessing these demand side flexibilities in an integrated energy system. The outputs of MSP1 will include total system costs, total carbon emissions, total use of each fuel type and any external costs and benefits that accrue to players outside of the particular energy systems being studies.