There is little published information on ageing criteria for Juvenile Manx Shearwaters Puffinus puffinus. We detail ageing criteria that have been used at Copeland Bird Observatory, Co. Down, for approximately 30 years. It involves using basic moult techniques and differences in feather colour, shape and pattern to distinguish fully moulted juvenile birds from adults. Juvenile birds have black feathers, a distinctive pointed and hooked primary shape, and pale edgings to their mantle feathers. Adults have brown feathers with rounded, worn primaries and mantle feathers. The shape and colour of the axilliaries have been reported as an ageing criterion and these were found to be useful features. Using these techniques, it is possible to confidently age fully grown down-less Manx Shearwaters caught in the autumn. These characters may also be of use in identifying first year birds found away from the colony in the year after fledging.