The empirical white dwarf cooling sequence

White dwarf stars cool as they age, hence they can be used as chronometers for various stellar systems. Their value as clocks depends critically on how well we understand the physics of cooling. In this contribution we outline how to derive an empirical cooling sequence and then compare it with a standard white dwarf cooling model. Some differences are noted suggesting that there is still missing physics in the models. A more detailed version of this communication can be found in Goldsbury et al. (2012).