The forced oscillation technique has recently been modified for use in children. The technique involves a child breathing an oscillating airflow at the resonant frequency of the respiratory tract. At this frequency, the reactive components of respiratory impedance are equal and opposite in sign, and it is therefore possible to measure directly the resistive component of respiratory impedance. This is known as the total respiratory resistance. Full descriptions of the theory, apparatus and method involved in the technique are given with a discussion of the common artefacts encountered. Examples of typical clinical applications are included to illustrate the usefulness of the technique.