A computer simulation tool, named "CHAMPS-Multizone" is introduced in this paper for analyzing bothenergy and IAQ performance of buildings. The simulation model accounts for the dynamic effects ofoutdoor climate conditions (solar radiation, wind speed and direction, and contaminant concentrations),building materials and envelope system design, multizone air and contaminant flows in buildings,internal heat and pollutant sources, and operation of the building HVAC systems on the buildingperformance. It enables combined analysis of building energy efficiency and indoor air quality. Themodel also has the ability to input building geometry data and HVAC system operation relatedinformation from software such as SketchUp and DesignBuilder via IDF file format. A "bridge" to accessstatic and dynamic building data stored in a "virtual building" database is also developed, allowingconvenient input of initial and boundary conditions for the simulation, and for comparisons between thepredicted and measured results. This paper summarizes the mathematical models, adoptedassumptions, methods of implementation, and verification and validation results. The needs andchallenges for further development are also discussed.