The population-based reference interval is the most widely used tool for interpretation of individual patient laboratory test results. The clinical value of those results depend crucially on the reference intervals with which they are compared, and all efforts directed at ensuring analytically precise and accurate test results are, to a greater or lesser extent, in vain if the relevant reference interval is invalid or deficient in some way. This is the first of two introductory articles focusing on the reference interval. Here we consider the concept of the reference interval as an interpretive tool, define some relevant terms and address some of the theoretical aspects to be considered when constructing and using reference intervals. The second article - An introduction to reference intervals (2) - some practical considerations - which lends heavily on current expert opinion contained in a 2008 report (C28-P3) from the Clinical and Laboratory Standards Institute, describes how to validate reference intervals according to internationally agreed standards. Construction of a reference interval from scratch according to these standards is an enormous undertaking that could not reasonably be expected to be within the remit of most clinical laboratories. By contrast, validation of an existing reference interval is a much less arduous task that can and should be undertaken by all laboratories when introducing an established test to the laboratory repertoire or when making changes to existing test methodology/sampling protocols.