In particular, physiological aspects of breath testing are covered with emphasis on factors influencing the precision and accuracy of results. The increasing use of punishable limits of blood and/or breath alcohol concentration makes chemical test evidence a popular target for defense attack and litigation in trials concerned with driving under the influence (DUI). Historical developments in theory and application of breath testing as evidence of intoxication are briefly outlined. The absorption, distribution, and elimination of alcohol in the human body are covered as background for understanding the passage of alcohol from blood to breath. Research on the blood/breath alcohol ratio and the factors that influence this relationship including mouth-alcohol, regurgitation, breathing technique, arterio-venous differences, blood hematocrit value, pulmonary disease, body temperature, expired air temperature, and temperature and humidity of ambient air are critically evaluated and discussed. Both blood-alcohol and breath-alcohol measurements are suitable to provide objective evidence of alcohol load in the organism and the associated impairment of driving skills. With per se statutes, the magnitude of sampling and analytical errors inherent in methods of analyzing alcohol for legal purposes must be carefully documented. The final prosecution result can be adjusted to allow for uncertainties in the analytical procedures used. 3 figures, 6 table, and 107 references (Author abstract)