Stromatolites are now used more and more frequently in attempts to solve Precambrian stratigraphic problems. Abundant new data that have become available during the last two years support the view that the methods are valid and useful, provided that rigorous taxonomic procedures are followed. Contrary to earlier indications that Early and Late Proterozoic stromatolites are identical, none of the 53 forms ('form-species') now known from the Early Proterozoic are also reported from younger rocks. Nonetheless, it has become increasingly apparent that correlations based only on group ('form-genus')-level identifications can be misleading. The problems of establishing identity at the form-level indicate the need to substantially improve descriptive methods, and in particular to make more use of numerical techniques. Stromatolite assemblage zones distinguished in Middle to Late Proterozoic sequences are 100-350 Ma in duration, with boundaries at 1650 ± 50, 1400 ± 50, 1050 ± 50, 650-680 ± 20 and 570 ± 20 Ma.