BRE specialists undertook a review of the key parameters that determine indoor air quality (IAQ) and proposed a draft protocol for undertaking an IAQ assessment. Comments from experts were received and a field trial undertaken. This report provides the final protocol as revised following the field trial. The aim is for this report to be used as guidance by specialists undertaking an IAQ assessment and by building owners/managers concerned about indoor air quality. The key parameters identified are temperature, humidity, non-biological particles and fibres, biological particles, radon, other inorganic gases, organic compounds and gases associated with landfill sites (eg methane). These are discussed in appendices to provide information about the possible health effects, available guidelines for acceptable concentrations, the appropriate measurements and the methods of measurement. 50 pages.