This thesis, authored by Per Sahlin from the Department of Building Sciences at the Royal Institute of Technology in Stockholm, Sweden, focuses on the design of a general-purpose simulation environment called IDA and the Neutral Model Format (NMF), a program-independent language for modeling dynamical systems using differential-algebraic equations with discrete events. The primary goal is to address the limitations of monolithic simulation programs, which are difficult to upgrade and maintain over the long term. IDA and NMF are designed to facilitate the development of user-friendly, specialized GUI-based tools that are easy to use for end users without simulation expertise and have good prospects for long-term maintenance and reuse. The thesis includes seven introductory chapters and seven separate papers, covering the design issues of IDA and NMF, historical overviews of selected applications, and future trends in NMF. The thesis highlights the importance of building simulation in the building industry, noting that traditional monolithic tools are inadequate for the diverse and evolving needs of designers. It discusses the challenges in technology transfer, access to appropriate tools, integration into design environments, and validation of simulation results. The thesis also addresses the problems of model reuse, high development costs, and the limitations of existing special-purpose tools. The primary target applications include building loads and energy calculation, multizone air flow, coupled thermal and fluid flow problems, and demand-controlled ventilation. The thesis provides a detailed review of related projects and tools, such as TRNSYS, ESACAP, HVACSIM+, and SANDYS, and evaluates their suitability for building simulation. It concludes with a discussion of the applicability of modular simulation environments (MSEs) as a replacement for traditional monolithic tools, emphasizing the efficiency of the application development process and the general usability of the developed applications.This thesis, authored by Per Sahlin from the Department of Building Sciences at the Royal Institute of Technology in Stockholm, Sweden, focuses on the design of a general-purpose simulation environment called IDA and the Neutral Model Format (NMF), a program-independent language for modeling dynamical systems using differential-algebraic equations with discrete events. The primary goal is to address the limitations of monolithic simulation programs, which are difficult to upgrade and maintain over the long term. IDA and NMF are designed to facilitate the development of user-friendly, specialized GUI-based tools that are easy to use for end users without simulation expertise and have good prospects for long-term maintenance and reuse. The thesis includes seven introductory chapters and seven separate papers, covering the design issues of IDA and NMF, historical overviews of selected applications, and future trends in NMF. The thesis highlights the importance of building simulation in the building industry, noting that traditional monolithic tools are inadequate for the diverse and evolving needs of designers. It discusses the challenges in technology transfer, access to appropriate tools, integration into design environments, and validation of simulation results. The thesis also addresses the problems of model reuse, high development costs, and the limitations of existing special-purpose tools. The primary target applications include building loads and energy calculation, multizone air flow, coupled thermal and fluid flow problems, and demand-controlled ventilation. The thesis provides a detailed review of related projects and tools, such as TRNSYS, ESACAP, HVACSIM+, and SANDYS, and evaluates their suitability for building simulation. It concludes with a discussion of the applicability of modular simulation environments (MSEs) as a replacement for traditional monolithic tools, emphasizing the efficiency of the application development process and the general usability of the developed applications.