The aim of the work presented in this paper is to develop a common scientific methodology for the assessment of reliability of the pipeline network energy systems. The developed methodology is applicable for district heating, gas and oil supply networks. The reliability of all these systems depends on the degradation mechanisms and structural integrity of pipes. Structural integrity is influenced by loadings, defined by thermal-hydraulic processes. The reliability of energy network systems also depends on reliability of the installed equipment and the supply source. The reliability of pipeline networks is treated as a complex task. Consequently, the developed methodology includes three types of analyses: probabilistic mathematical, deterministic thermal-hydraulic and deterministic-probabilistic structural integrity analyses. As a pilot study, the application of the developed methodology for the analysis of Kaunas (Lithuania) district heating system was performed. The main results of this pilot study are presented in the paper. Although the original intention is to apply the developed methodology for the assessment of the reliability of the Lithuanian energy systems, in general, the methodology is applicable for any pipeline network system in any country. (c) 2012 Elsevier Ltd. All rights reserved.