Measuring Mass: From Positive Rays to Proteins is part of a celebration of fifty years of the Annual Conference on Mass Spectrometry and Allied Topics. As such, it is intended not only for practitioners of mass spectrometry but also for the lay reader interested in knowing more about the field. Many who practice the art and science of mass spectrometry are unaware of how the technique is applied outside their particular area of expertise. This short exposition will provide the practitioner and lay reader alike with an appreciation for the diverse applications of mass spectrometry in present-day scientific endeavors. Measuring Mass is also intended to celebrate the major events in the history of mass spectrometry.