Dating is the process of determining the age of a fossil specimen.

There are two types of fossil dating: 
  • Absolute dating where the age of the fossil is determined, and
  • Correlative dating where the surrounding rock is dated. 
Sometimes correlative dating is preferred over absolute dating, particularly with microfossils because the methods for absolute dating would destroy the fossil specimen.
The most common method of absolute dating is carbon dating. This measures the amount of the radioactive isotope carbon-14 (14C) in the sample. There is a certain amount of carbon-14 in all organic matter, and over time it breaks down into carbon-12 or regular non-radioactive carbon. The less carbon-14 in a fossil, the older it is. Unfortunately carbon dating is only accurate to 50,000 years with an error of ±500 years. Sometimes oxygen-15 (15O) is used instead. 
One key problem with isotopic dating (carbon dating) is that it assumes that carbon-14 breaks down to carbon-12 at a constant rate. Whilst there is no evidence to show that it doesn't, there is nothing to say that other factors may speed up or slow down decay over extended periods.
The other methods of dating use uranium or plutonium. These are radioactive elements that have a much longer half-life. These are used to measure fossils older than 50 000 years of age.