Lichenometry uses the size/age relationship of lichens to date stone surfaces. A study of neolithic stone circles at Rollright in Oxfordshire and Castlerigg in Cumbria showed that stone movements in the recent past may he deduced from the varying sizes of lichen populations on different surfaces and approximate dates may be suggested for periods of change. Thalli of Aspicilia calcarea dating to around 1366 and of Rhizocarpon geographicum to 1523 are reported. Problems in past work have mainly concerned the effects on growth of environmental variation, anomalous growth, colonization rates, aspect preferences, variable lifespans and an inconsistent methodology. Where a consistent methodology can be applied the technique shows considerable potential not only for dating in glacial environments, its major use to date, but also for assessing the detailed treatment over the recent centuries of historic or prehistoric structures. ADDITIONAL KEY WORDS: Colonization ~ environment - growth - lichens ~ variation - weathering.