Biological monitoring usually relies on the collection of blood and urine samples. Although being non-invasive and providing an inextinguishable sampling pool, the analysis of exhaled breath is not well established. A gas phase measurement is, however, inherently simpler than the analysis of complex biological fluids, and modern methods have identified hundreds of volatile compounds in the breath of persons exposed to normal environmental concentrations. The most commonly deployed analytical techniques in breath analysis are gas chromatography combined with mass spectrometry (GC/MS) and other MS-based methods. Lately, also laser-based optical methods, such as cavity ring-down spectroscopy (CRDS), have emerged in the field. With such instruments, it is possible to accurately quantify the concentrations of volatiles in exhaled breath down to below part-per-billion (ppb) levels with sub-second time resolution. Laser spectroscopy thereby enables real-time investigations during and after exposure to exogenous chemicals. In general, depending on the sampling approach used, the measured levels of the breath compounds may vary significantly. It is therefore of importance to systematically study and account for the phenomena affecting the recorded concentrations, and subsequently select an appropriate sampling and measurement strategy. In Helsinki, we have used CRDS to study the background levels of hydrogen cyanide (HCN), ammonia (NH3) and acetylene (C2H2) in the exhaled breath of healthy volunteers. Different sampling techniques have been employed in an effort to standardize the breath sampling event. The realtime elimination kinetics of breath C2H2 after smoking has also been studied.