Measurement of biologically important effector protein molecules has been a long-standing essential component of biological research. Advances in biotechnology, in the form of high-resolution mass spectrometers, and in bioinformatics, now allow the simultaneous quantitative analysis of thousands of proteins. While these techniques still do not allow definitive identification of the entire proteome of complex mixtures, such as cells, quantitative analyses of hundreds to thousands of proteins in such complex mixtures provides a means to elucidate molecular alterations that occur during perturbation of cellular systems. This article will outline considerations of reducing sample complexity, by strategies such as multidimensional separations (gel-based and chromatography-based, including multidimensional protein identification technology). In addition, some of the most common methods used to quantitatively measure proteins in complex mixtures (2D difference in-gel electrophoresis, isotope-coded affinity tags, isotope-coded protein labeling, tandem mass tags, isobaric tags for relative and absolute quantitation, stable isotope labeling of amino acids in cell culture and label-free), as well as recent examples of each strategy, are described.