To estimate annual changes and trends in the population of informal carers and to investigate transitions to caregiving by age, gender, locus of care, and level of involvement. Longitudinal analysis of data from the British household panel survey, 1991 to 1998, an annual prospective survey of a nationally representative sample of more than 5000 private households in England, Scotland, and Wales. Over 9000 adults over 16 years interviewed personally in successive waves of the survey, including around 1300 informal carers each year. One third of co-resident carers and 40% of extra-resident carers start caregiving each year and similar proportions cease to provide care. Five year period rates are at least 75% higher than the one year prevalence estimates. Almost everyone is involved in caregiving at one time or another and over half are likely to provide 20 hours or more care per week at some point in their lives. Recent trends indicate that more adults are becoming heavily involved in providing longer episodes of care. Although the onset of caregiving peaks in late middle and early older age, above average incidences span three decades or more of adult life. Age variations in the start of caring relationships are driven by the changing demands for care within and between generations over the life course. There is no firm evidence that carers increase their involvement in caring activities over the first three years of a caring episode. The population of carers is constantly changing as some people stop providing care and others take on a caring role or vary their level of involvement. Policy measures responsive to the diversity of caring roles, and geared around key transitions, are likely to be most effective in supporting carers through changing circumstances. Recognition and support for carers who are heavily involved in caring activities from the outset should be a priority.