Model-based reasoning focuses on formal theories and algorithmic techniques for system-wide diagnosis, planning, and optimization of technical systems.  Distinctive features include the use of compositional models to capture the behavior of components and their interaction, and a generic set of reasoning algorithms.  This talk highlights some of our research in the area of model-based systems, particularly to expand towards on-line estimation and control and hybrids of constraint-based and probabilistic models to capture software behavior and continuous dynamics, and presents some recent applications in the domain of energy informatics and sustainable mobility.