Tracer-gas techniques have become widely used to measure the ventilation rates in buildings. A tracer gas is an idealized substance used to tag volumes of air so as to be able to infer their bulk movement; the properties of a perfect tracer are discussed. The basic principle involved is that of canservation of mass (of both air and tracer gas) as expressed in the continuity equation; by monitoring the injection and concentration of the tracer, one can infer the exchange of air. This report will summarize the techniques in use. Although there is only one continuity equation, there are many different experimental injection strategies and analytical approaches. These different techniques may result in different estimates of ventilation due to uncertainties and biases of the procedures. This report will analyse the spectrum of techniques, including some of the relevant error analyses.