A method of monitoring an immunoassay comprises determining the position of the flow edge of fluid across an immunoassay region. This allows, for example, early detection of possible errors in the test. For example the speed of the movement of the fluid flow edge may be determined from measurements of the position of the fluid flow edge at times throughout the test, and the determined speed may be compared with one or more thresholds to determine whether the test is proceeding correctly. A further method of monitoring an immunoassay comprising monitoring the variation with time of a reference zone of an immunoassay region. This can provide the user with early information as to whether the test is proceeding correctly. The above methods may also be used to provide methods to improve the run conditions of a test during its development. The method may be applied to a competitive assay.