The interrupter technique (Rint) is a quick, easy and effort-independent way to obtain indirect measurements of airways resistance in the preschool child. Results may be obtained using a portable Rint machine or a whole-body plethysmograph. Normative data are available and recent studies have improved standardisation of the methodology. Despite this, between-occasion results can be variable, particularly in children with wheeze. This limits the usefulness of the test in the assessment of long-term interventions such as the administration of inhaled steroids. The most useful role for Rint, therefore, appears to be in the assessment of bronchodilator responsiveness where it is as sensitive as spirometry in separating children with reversible airways disease from healthy controls. This paper describes the physiology of the interrupter technique and the methodology needed to obtain reliable results. Normal ranges are provided. The clinical applicability, repeatability, strengths and limitations of the technique are also discussed.