The UKERC Systems Theme has played an important role in the development of the UK's capacity to think systematically about the future of the energy system. Key tools in this process have been the development of scenarios, and the development and use of the MARKAL energy system model. This project reflects on scenarios and on the use and communication of MARKAL, with a view to informing future UKERC work. Specifically, the project conducted retrospective analysis of pre-UKERC energy scenarios for the UK (published from 1977-2002), examined the scenarios produced by the UKERC systems theme, and studied the use and communication of the UK MARKAL model. The diversity of scenario methods and approaches developed within UKERC is valuable, and should be fostered further. Too narrow a range of techniques and teams developing scenarios would risk constraining the ability of UKERC to open up thinking to a wide range of possibilities, perspectives and framings, which history suggests is important. UKERC scenarios have tended to be dominated by futures in which mitigation goals are met, and in which scenario differences are driven by policy or technology, though there are of course exceptions. As UKERC Phase 3 begins, there is a case for reflecting further on the range and type of uncertainties addressed within energy system scenarios, and the diversity of tools and techniques used to generate them. A core tool of the UKERC systems theme has been the UK MARKAL model. The research undertaken for this project indicates that MARKAL has generally been used and communicated appropriately, in part because of good working relationships between government analysts and UKERC researchers. There are also areas in which there is room for improvement, and UKERC Phase 3 provides an opportunity to learn the lessons from previous experience.