The association between birth cohort and subsequent mortality has been of interest especially following publication of studies around 1930 of cohorts born up to the latter part of the nineteenth century, particularly for England and Wales. Updated results are presented for this population, together with those for two other cohorts, twentieth-century Japanese and British populations born about 1930, which have been identified as having particularly clear-cut birth cohort patterns, and which are used to underpin incorporation of cohort effects in both British official and actuarial mortality forecasts. Graphical methods used to identify cohort patterns are discussed. A number of limitations and difficulties are identified that mean that the conclusions about the predominance of cohort effects are less robust than often assumed. It is argued that alternative explanations should be considered and that the concentration on birth cohorts with particularly advantaged patterns may distort research priorities.