Dendrochronology is the study and dating of annual rings in trees. Each year a tree adds a layer of wood to its trunk and branches thus creating the annual rings we see when viewing a cross section. New wood grows from the cambium layer between the old wood and the bark. In the spring, when moisture is plentiful, the tree devotes its energy to producing new growth cells. These first new cells are large, but as the summer progresses their size decreases until, in the fall, growth stops and cells die, with no new growth appearing until the next spring. The contrast between these smaller old cells and next year's larger new cells is enough to establish a ring, thus making counting possible.

Conifer and angiosperm trees possess trunk structures and lay down wood differently to one another. Conifer tree rings consist of early wood (lighter in colour made up of large diameter cells with thin walls) and late wood (darker made of small cells with thick walls). Angiosperms are slightly different in that you generally look for changes in diameter of vessels in the wood tissues; early wood having larger diameter vessels than late wood.

Although trees essentially lay down annual rings, it is not sufficient to simply count the rings to get an accurate age of a single tree. Cross-dating is the most commonly used method by dendrochronologists. By taking a number of samples from trees in an area, their ring patterns can be matched up to get an exact date (to the year) a particular ring is formed. Skeleton plotting is a common cross-dating method whereby tree ring width variation is plotted on graph paper (or by computerised graphs) to visualise and match up variation patterns in samples. Cross-dating can help overcome errors in dating from simply counting rings in a single sample. A sample can possess locally absent rings and/or false rings/bands that can give inaccurate results.

Dendrochronology is now used in a variety of areas of science and archaeology; not simply for the aging of a particular tree. Variation patterns in rings can give hints to the climate a tree has experienced - e.g. reduced rings during periods of drought. This is particularly useful in areas where there are few weather stations and regular records rarely go back further than 100 years. Dendrochronology on trees from Alaska, Scandinavia and Siberia have been of particular interest for scientists investigating climate change as it is thought global warming will have greatest effect on ecosystems nearer the poles. Dendrochronology is now becoming a hugely important tool for archaeologists. Radiocarbon dating was once the only scientific method for aging wooden objects. However C14 analysis can only be accurate at best plus or minus 20 years and even up to 2 centuries. In contrast dendrochronology can date wooden artefacts to the exact year, even season the tree was felled.