It is based on the fact that radiocarbon 14 C is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. These techniques can allow measurement of dates up to 60, and in some cases up to 75, years before the present. Because the radiocarbon is radioactive, it will slowly decay away. There are several other possible sources of error that need to be considered. The ratio of 14 C to 12 C is approximately 1. Older dates have been obtained by using special sample preparation techniques, large samples, and very long measurement times.
The simplified approach described above does not tell the whole story.
Cool examples of radiometric dating?