Modelica is a modern, strongly typed, declarative, equation-based, and object-oriented language for modeling and simulation of complex systems. Major features are: ease of use, visual design of models with combination of lego-like predefined model building blocks, ability to define model libraries with reusable components, support for modeling and simulation of complex applications involving parts from several application domains, and many more useful facilities. This paper gives an overview of some aspects of the Modelica language and the OpenModelica environment – an open-source environment for modeling, simulation, and development of Modelica applications. Special features are MetaModeling for efficient model transformations and prototype generation of parallel code for multi-core architectures. Ongoing work also include UML-Modelica integration and interoperability support through the new FMI standard.