Social mobility has become a topic of central political concern. In political and also media circles it is widely believed that in Britain today mobility is in decline. However, this belief appears to be based on a single piece of research by economists that is in fact concerned with intergenerational income mobility: specifically, with the relation between family income and children's later earnings. Research by sociologists using the same data sources – the British birth cohort studies of 1958 and 1970 – but focusing on intergenerational class mobility does not reveal a decline either in total mobility rates or in underlying relative rates. The paper investigates these divergent findings. We show that they do not result from the use of different subsets of the data or of different analytical techniques. Instead, given the more stable and generally less fluid class mobility regime, it is the high level of income mobility of the 1958 cohort, rather than the lower level of the 1970 cohort, that is chiefly in need of explanation. Further analyses – including ones of the relative influence of parental class and of family income on children's educational attainment – suggest that the economists' finding of declining mobility between the two cohorts may stem, in part at least, from the fact that the family income variable for the 1958 cohort provides a less adequate measure of ‘permanent income’ than does that for the 1970 cohort. But, in any event, it would appear that the class mobility regime more fully captures the continuity in economic advantage and disadvantage that persists across generations.