Computational fluid dynamics (CFD) is used routinely to predict air movement and distributions of temperature and concentrations in indoor environments. Modelling and numerical errors are inherent in such studies and must be considered when the results are presented. Here, we discuss modelling aspects of turbulence and boundary conditions, as well as aspects related to numerical errors, with emphasis on choice of differencing scheme and computational grid. Illustrative examples are given to stress the main points related to numerical errors. Finally, recommendations are given for improving the quality of CFD calculations, as well as guidelines for the minimum information that should accompany all CFD-related publications to enable a scientific judgment of the quality of the study.