Abstract: An assay device for detecting the presence of an analyte in a sample, wherein a visible signal indicative of the presence or absence of said analyte is produced at a detection site on a support, characterised in that said signal is generated or enhanced by means of a signal enhancement reaction between a labelled first binding reagent which is labelled and a label developing means, which are arranged to be delivered to the detection site in a single assay step but in a sequential manner such that the first binding reagent arrives at the detection site ahead of the label developing means. The sequential delivery of the reagents to the detection site is provided by techniques such as liquidic circuits, slow release agents, and others. Methods of carrying out assays and kits for use in the assays form a further aspect of the invention.