Recent years have seen great upward leaps in the development of mass spectrometry applied to the field of proteomics. Today it is possible to take a complex biological sample such as organelles, cells, tissue or a biofluid, perturbed or stimulated in some way, and identify and quantitate up to several thousand proteins and determine the level of relative change caused by the perturbation or stimulus. The current challenge is not to identify or quantitate proteins in a limited set of samples, but to profile large series (clinical samples, time-course, sub-cellular compartments) at sufficient depth, and to interpret and make biological sense of the data.