When the last edition of this book was published in 2000, the field of proteomics was in its infancy. At that time, multidimensional liquid chromatographic separations were being introduced as an alternative to traditional gel-based techniques for separating complex protein and peptide mixtures prior to mass spectrometric detection. Today, this approach – referred to as shotgun proteomics – is considered routine for lar- scale global analyses of protein mixtures. Now in its adolescence, proteomics is fundamentally transforming biological and medical research. Much of this transformation can be attributed to technological advancements, particularly in mass spectrometry. Much wider accessibility of hi- resolution and mass measurement accuracy instrumentation in recent years has ini- ated a new revolution in the field by providing more reliable data and shifting the focus from cataloging proteins to precisely quantifying changes in protein abundance over time and in response to stimuli. Advanced mass spectrometers and novel ion d- sociation schemes such as electron transfer/capture dissociation make it possible to venture boldly into the maze of protein posttranslational modifications, which are an integral component of understanding functional proteomics in the spatial and t- poral domains. Another area that has benefited from these advancements is top-down proteomics, an emerging method essential for characterizing various protein variants that has potentially high impact in biomedical research.