The energy field has been dominated in recent years by two different themes: on the one hand, the integrated approach to energy planning, and, on the other hand, the ownership structures for industries' management. This thesis investigates a broad range of problems arising in relation to these themes, including demand-side management, technology diffusion and supply-side management. It also reviews the corresponding approaches to assist the decision making process in connection with these problems. It is shown here that there is a methodological vacuum with respect to the support tools for energy analysis due to emerging policy challenges in the energy field, and methodological solutions are proposed to this end. The contribution is thus primarily three fold: 1) a methodology to assist both policy analysis and strategic processes, 2) a systems platform to assess policy and strategy, and 3) evaluation of specific policy and strategic issues arising in the new liberalised environments being implemented in both the British and Colombian energy systems. In this case, methodological connections are appropriate as the Colombian Government has incorporated aspects of the British energy system set-up. This thesis contains seven chapters. Chapter 1, from an extensive literature review, establishes a dilemma in relation to the methodology required to support system analysis and planning. Chapter 2 presents a new methodological proposal to meet the requirements. Chapter 3 concept-tests the proposal and specifies an analysis-support platform as a generic aid to modelling. Chapter 4 develops a case study for the UK energy system, partially testing the proposed approach and the analysis platform especially constructed for this situation. Chapter 5 elaborates upon a case study for the Colombian energy sector, examining in detail both methodology and the analysis platform uniquely designed for this case. Chapter 6, with the support of a platform construct, studies and assesses energy policies for Colombia. Finally, Chapter 7 summarises and concludes the major findings of this thesis.