The dissertation investigates the problem of balancing service quality and delivery costs by applying modularity principles. The object of research is decision support for service modularization. The dissertation aims to develop a decision support tool for service modularization that ensures rational decision-making in planning modular service delivery and creates the conditions for balancing service quality and cost. The dissertation consists of the introduction, three chapters, general conclusions, lists of references, the author’s publications, and eleven annexes. The first chapter analyzes the emergence of the service notion and proposes a new service notion that meets the expectations of modern management. It also describes the modular design in the service context and identifies the main problems in its application. After examining the concept of the service process, the technological aspects related to the service modularity are highlighted. The second chapter investigates the relationships between the premises and the results of service modularization, and the content of planning activities related to service modularization. Based on the obtained results, the purpose statement of service modularization is presented. Further, the work outlines the four main areas of decision support for service modularization. The discussion of the existing methodological potential for these areas of decision support notes the lack of a methodological framework for assessing the level of service modularization. The chapter closes with a practical tool for decision support in service modularization. The third chapter underpins the empirical research and presents the methodology for assessing the level of service modularity. The specified content of the service modularity construct led to creating a three-dimensional scale, supported by a linguistic description of modularity levels. Also, the chapter offers a list of evaluation criteria and an instrument for collecting qualitative data. The qualitative data collection instrument was tested in a pilot study, and then, three service organizations in Lithuania were involved in an internal expert assessment and determining the level of their service modularity. The chapter ends by discussing perspectives for the practical use of the decision support tool in service modularization and identifying further research needs. Six publications on the dissertation topic were issued in peer-reviewed scientific publications. Two of them are included in the Scopus database. The research results were presented at seven scientific conferences in Lithuania and abroad.