The Doctoral Thesis presents a set of qualitative and quantitative studies on trends and problems of the district heating sector development with the aim to quantify possible development scenarios of the DH systems. Optimisation of energy systems has been previously studied by A. Borodiņecs, E. Dzelzītis, G. Klāvs, A. Krēsliņš, M. Aberg, J.E. Christensen, J. Laanearu, H. Lund, etc. The goal of the Doctoral Thesis is to develop a methodology and planning tool for comprehensive evaluation of district heating system efficiency within different development scenarios. To achieve this goal, the following tasks have been set: 1) to analyse trends and challenges of the current energy policy of the European Union; 2) to study the future development potential of DH systems; 3) to perform studies on reduction of thermal energy consumption as a result of renovation of DH networks and existing dwellings; 4) to develop the methodology for DH system modelling and evaluation; 5) to develop a dynamic simulation tool for modeling of DH system heat and mass transfer processes and quantitative evaluation of DH system efficiency on the basis of the proposed methodology; 6) to validate the developed tool by comparing the predicted DH system specific parameter values with the measured ones; 7) to simulate the proposed DH system development scenarios and select the most advantageous ones based on evaluation criteria values. Scientific Novelty. The model with multiple integrated heat and mass transfer processes has been developed on the basis of the proposed methodology for the modeling and assessment of the 4th generation DH systems under changing thermal energy consumption. Practical Application. The developed methodology and district heating planning tool are practically applicable by planning and development departments of heat supply system operators and municipalities for evaluation and increase of the efficiency of DH systems. The Doctoral Thesis has been written in English. It contains the introduction, 4 chapters, conclusions, bibliography with 103 reference sources. The Doctoral Thesis has been illustrated by 51 figures. The volume of the Thesis is 108 pages. The results of the Doctoral Thesis have been reported in 10 international conferences and published in 11 scientific publications.