Recent advances in large-scale DNA sequencing technology have made it possible to sequence the entire genome of an organism. Attention is now turning to the analysis of the product of the genome, the proteome, which is the set of proteins being expressed by a cell. Two-dimensional gel electrophoresis can be used to create cellular protein maps which give a quantitative and qualitative picture of the proteome. Mass spectrometry is the method of choice for the rapid large-scale idenfification of these proteomes and their modifications. An understanding of these methods is critical for scientists in the "Post-Genome" era.