BRE specialists reviewed the parameters that determine indoor air quality (IAQ) and drafted a protocol for undertaking an IAQ assessment. This report provides the final protocol, revised following comments from experts and a field trial, and is intended as guidance for specialists undertaking an IAQ assessment and building owners/managers concerned about IAQ. The key parameters are temperature, humidity, non-biological particles and fibres, biological particles, radon, other inorganic gases, organic compounds and gases associated with landfill sites (eg methane).