The recent publication of `Guidelines for assigning species rank', by the Taxonomic Sub-committee (TSC) of the BOURC (Helbig et al. 2002), provided a long-awaited `written constitution' for birders interested in the taxonomic issues surrounding the British List. Some of its main points are summarised here. Perhaps surprisingly, the TSC has not formally adopted any particular species concept, but has defined two conditions that a taxon (in most cases the taxon being considered will be one currently recognised as a subspecies) must fulfil if it is to be assigned to species rank. These conditions follow most closely the Evolutionary or General Lineage Species Concepts (Mayden 1997; de Queiroz 1999); they will form the practical basis of TSC decisions, and are broadly as follows:1. Species must be diagnosable: individuals of each taxon must be absolutely distinguishable from individuals in all other taxa. The TSC has defined explicit criteria of diagnosability, in order to make such decisions less arbitrary than may have been the case in the past. 2. Species must have diverged sufficiently, so that they are not likely to merge with other (similar) species in the future: they are expected to maintain their genetic and phenotypic integrity.