First, we must establish what a fossil is. A fossil is the remains or traces of an ancient life form, which have undergone mineralization. This means that a true fossil contains virtually none of the original creature's material. Consequently true fossils cannot be dated with the Carbon 14 method. Carbon 14 dating is only good to approximately 60,000 years in the past, and can only be done on materials that still contain their original carbon based matter. It cannot be used at all on trace fossils such as footprints.
Fossils are often categorized by relative age, where the placement of the fossil in relation to other fossils helps to determine its place in the sequence of life on the planet. In order to calculate an absolute age, which establishes how many years in the past the life form actually lived, is rather complex, and is expensive. Radioactive dating is an important tool; rather than being a single method, this term refers to a collection of related tests that can be applied; the choice of test is determined by the relative age of the fossil, its location, and by what type of fossil it is. Generally scientists must theorize, based on the fossil itself, what radioactive isotopes it may contain, and then those are evaluated to see how much of the radioactive form of the isotope remains, and how much has degraded into a stable form. The ratio and the known half-life of the radioisotope are used to compute an absolute age.
A tool known as the electron microprobe has been developed which allows pinpoint analysis of element makeup. This tool has made the use of radiometric dating simpler and faster.
Scientists use a method like carbon-14 dating to ascertain the age of certain fossils. When these organisms were alive they incorporated a radioactive isotope of carbon into their systems in a measurable ratio. When they died, the carbon-14 that remained in their system began to degrade to regular carbon-12. The half-life of carbon-14 is a measurable quantity that can determine the number of years that organism is old. So all the scientists have to do is measure the amount of carbon-14 nuclei still remaining in the organism, calculate the number of halflives it has gone through, and calculate the number of years old that fossil represents. This is a very reliable method of dating organisms that have trace amounts of carbon-14 still remaining in their systems. For fossils older than the halflife of carbon-14, other substances and methods have to be employed, that have longer halflives than carbon-14.