Respiratory gas analysis as an indicator for metabolic strain during exercise has a long history. First introduced in the 18th century, huge gas collectors served for the determination of oxidative energy delivery. While still being accepted as accurate, this particular method delivers data of low temporal resolution only. Further developments of gas analysis techniques therefore focused on a higher density of data. When algorithms became available for indispensable calculations, the so-called "breath-by-breath" (BBB) method was established some decades ago. Thereby, the term BBB in the narrower sense means that a particular physiologic value is determined for each of a subject's single respiratory cycles. Reliable application of this approach depends on the performance of available computer systems, the quality of the analyzing software routines, and the responsiveness of the gas analyzers. Thus, it appears that even nowadays technical progress is continuing in this area. This review describes technical aspects and prerequisites of the BBB approach and its specific areas of application.