Abstract: The yeast and human spliceosomes represent the first two multiprotein complexes of which protein components were identified solely by mass spectrometry (MS). In this chapter, the different approaches used for the purification of these protein complexes, the MS analysis of the components, and some functional characterization strategies adopted are discussed. Even though from the time of analysis up to 2005 much has been achieved in terms of purification techniques, MS protein analysis and sequence information in public databases, the key knowledge gained from the very early complex analyses still hold true today. The analysis of protein complexes is a powerful approach for understanding the organization of proteins and how they act in units to exert their biological effects. The analysis also creates hypotheses for the role of novel proteins in the context of the cellular function of the protein complex under study. However, the work on the spliceosomes described in this chapter also illustrates the relative ease of protein identification by MS and the difficulty to provide detailed functional information for the vast amount of data generated in such a study.