AixLib is an open-source Modelica model library for building performance simulations. It is being developed at RWTH Aachen University, E.ON Energy Research Center, Institute for Energy Efficient Buildings and Indoor Climate (EBC) in Aachen, Germany. It contains Modelica models for building envelope and HVAC equipment such as boiler, radiator, heat pump and CHP. Emerging out of the Institute’s needs in research and teaching, AixLib contains models of different level-of-detail. In particular for building envelopes, it contains models focusing on single building scale (High Order Models) and for neighborhood or urban scale applications (Reduced Order Models). The library target applications are the investigation of components, entire, innovative Building Energy Systems and Urban Energy Systems. The library extends the collaboratively developed IBPSA library (https://github.com/ibpsa/modelica-ibpsa) , which focuses on basic model. To keep a high modelling quality, all models typically come with extensive examples and comprehensive documentation. AixLib is available under a 3-clause BSD-license. As the library is developed at RWTH Aachen University's EBC, the library's name AixLib is derived from the city's French name Aix-la-Chapelle, which the people of Aachen are very fond of and use a lot. With the name AixLib we follow this local tradition.