Climate change caused by human activities and their successful mitigation and adaptation to consequences is one of the crucial issues of humanity in the 21st century. One of the most common ways to tackle this issue is local and regional initiatives, which create energy plans for sustainable development. Sustainable energy planning at a local level is first time seen at some level, after the 1970s as a direct consequence of the energy crisis. A serious approach to sustainable local energy planning is noticed during the last 15 years, especially after European Commission started the Covenant of Mayor's initiative. The energy and climate goal of the European Union, till 2030 can be most easily achieved by planning sustainable development on a local level, due to a bottom-up approach, direct investments, and implementation. It is concluded that local governments have a crucial role in the mitigation of climate change. The most important steps in the development of local and regional energy plans are the analysis of the present situation considering energy consumption and GHG emissions, present and future energy and CO2 balance and estimation of reduction potential; a strategy to reach targets, with targets’ definition, measures, and implementation plan; and a regular plan for monitoring of the implementation and reaching the reduction goals. In this way, all phases in the process of local energy planning are included in the process. The main objective of this doctoral thesis is to upgrade the methods for local and regional energy planning through an optimal selection of measures and alternative scenarios for the reduction of CO2 emissions. The additional objective was to evaluate the possibility of mutual interaction and integration of measures through the development of predesigned sets of measures and to optimise the application of measures sets on local and regional levels considering economic, environmental, and social criteria. This thesis hypothesises is that with the use of optimisation models it is possible to find such sets of measures and alternative scenarios, which will with less financial means reach a minimal reduction of CO2 emissions by 40% in local and regional energy systems. With aggregation of local and disaggregation of regional plans use of financial means and available financing mechanisms for the implementation of measures can be optimised. The obtained results have shown that the optimisation of the measures can significantly influence the cost of mitigation measures needed for the fulfilment of the CO2 emission targets. The optimisation of mitigation measures can result in financial savings of up to three times than baseline scenario. In this way, it could be possible to increase the overall implementation of mitigation measures and avoid the problem of the lack of appropriate financial planning. Additionally, mutual interaction between measures, which influences the total result of energy consumption and CO2 emissions reduction, can have either synergetic, negative, or neutral interaction between them. The analysis showed the negative interaction is prevailing and that the scenario approach resulted in lower CO2 reduction potential than the individual assessment of each measure. It is recommended to use a scenario approach in the development of SECAP for the assessment of measures of CO2 reduction potential. This will provide more efficient planning of measures for the reduction in CO2 emissions on a local level and avoid overestimating the CO2 reduction potential when developing SECAPs. Finally, the analysis of the application of multilevel governance demonstrated that the alignment of energy targets and coordinated actions from different levels of governance is leading to the effective implementation of mitigation measures.