This guidebook is written by a working group of highly qualified international experts representing research, consulting and design.Both the development of CFD models for room air movement and progress in the general fluid dynamics research are strongly influenced by the increased computer power that has been available for the past few decades. CFD-calculations have rapidly been developed a powerful tool for the analysis of air and pollution distribution in various spaces. However, the user of CFD-calculation should be aware the basic principles of calculations, and specifically the boundary conditions. The reliability of the results depend very much on the skills of the person who performs the calculations. Guidebook is intended for users of CFD-programs, designers, manufactures and building owners. It gives also guidelines for those who order CFD-calculations from the experts. With its illustrations and examples it is also an excellent text book for universities and also the vocational training of various building professionals.