The acoustic output signal of orators and singers contains information from the voice source as well as from the articulatory organs. The extraction of formants to achieve information about voice timbre and vocal-tract configuration is among the essential methods in speech and language processing as well as in singing-voice research. However, the validity of formant analysis is limited by voice features and analysis parameters. The measurement of the acoustic impedance of resonating structures allows one to obtain all relevant information from their resonances. The design of a method for impedance measurement at the human lips by Joe Wolfe and John Smith dates back to 1995, and since then variations of the concept have been applied in fundamental singing-voice research (high soprano and tenor voices, overtone singing), as well as for investigations in other domains such as voice rehabilitation. This review article collects methods and applications over the last 25 years, giving reference to the many colleagues and students who have contributed with theses and supporting work. The presentation is accompanied by a demonstration of the method.