The tomographic atom probe (TAP) is the most advanced of the three-dimensional atom probes currently in use. Atom probes are unique in that both the lateral and the depth resolution are high. The general development of atom probes and the principle of operation of the TAP, in which the sample is field evaporated atomic layer by atomic layer, are outlined. The analysis of a nickel base superalloy is given as an example of an application in metallurgy.