We present an overview of the development and use of our selected-ion flow tube (SIFT) technique as a sensitive, quantitative method for the rapid, real-time analysis of the trace gas content of atmospheric air and human breath, presenting some pilot data from various research areas in which this method will find valuable application. We show that it is capable of detecting and quantifying trace gases, in complex mixtures such as breath, which are present at partial pressures down to about 10 parts per billion. Following discussions of the principles involved in this SIFT method of analysis, of the experiments which we have carried out to establish its quantitative validity, and of the air and breath sampling techniques involved, we present sample data on the detection and quantification of trace gases on the breath of healthy people and of patients suffering from renal failure and diabetes. We also show how breath ammonia can be accurately quantified from a single breath exhalation and used as an indicator of the presence in the stomach of the bacterium Helicobacter pylori. Health and safety applications are exemplified by analyses of the gases of the gases of cigarette smoke and on the breath of smokers. The value of this analytical method in environmental science is demonstrated by the analyses of petrol vapour, car exhaust emissions and the trace organic vapours detected in town air near a busy road. Final examples of the value of this analytical method are the detection and quantification of the gases emitted from crushed garlic and from breath following the chewing of a mint, which demonstrate its potential in food and flavour research. Throughout the paper we stress the advantages of this SIFT method compared to conventional mass spectrometry for trace gas analysis of complex mixtures, emphasizing its selectivity, sensitivity and real-time analysis capability. Finally, we note that whilst the current SIFT is strictly laboratory based, both transportable and portable instruments are under construction and development. These instruments will surely extend the application of this analytical technique into more areas and allow greater exploitation of their on-line and real-time features.