ESDU 71012 provides background information that will assist the user of the Series in the estimation of fluid forces on non-streamline (i.e. bluff) bodies. The Wind Engineering Series gives detailed numerical data for use in a wide range of problems in aeronautical@ chemical@ marine@ mechanical and structural engineering associated with the flow of fluid around bluff bodies. ESDU 71012 describes the physical phenomena for those who are not specialist aerodynamicists and want to understand the principles of the problem. It is mainly concerned with wind flow around bluff bodies@ such as buildings@ but the information is also applicable to other fluid flow problems such as in the marine environment.