MathModelica is an integrated interactive development environment for advanced object-oriented system modeling and simulation. The environment integrates Modelica-based modeling and simulation with graphic design, advanced scripting facilities, integration of program code, test cases, graphics, documentation, mathematical typesetting, and symbolic formula manipulation provided via Mathematica. The user interface consists of a graphical Model Editor and Mathematica notebooks. The Model Editor is a graphical user interface in which models can be assembled using components from a number of standard libraries representing different physical domains or disciplines, such as electrical, mechanics, block-diagram and multibody systems. The accessible MathModelica internal form allows the user to extend the system with new functionality, as well as to perform queries on the model representation and write scripts for automatic model generation. Furthermore, extensibility of syntax and semantics provides additional flexibility in adapting to unforeseen user needs.