The status of cytological and cytogenetic data as evidence of value in taxonomic decision-making and biosystematic investigations over the past 50 years is surveyed. The main headings considered are chromosome number, genome size, chromosome morphology, chromosome staining characteristics, chromosome and genome disposition in the cell, and chromosome behaviour and homology. The main conclusions are that many of the exciting new developments at the borders of cytogenetics and molecular biology (molecular cytogenetics) are producing a wealth of new data of enormous taxonomic and evolutionary importance, but that for these to reach their full impact it is essential that they are fully integrated with traditional cytological data, the need for which remains as great as ever.