A recurring theme in biology is the aggregation of mRNA-protein complexes (mRNPs) into higher order assemblies. Often these complexes play important roles in the regulation of gene expression, but the function of the conserved cytoplasmic mRNP assemblies - P bodies and stress granules, is not known. It is believed that the misregulation of granule assembly is related to disorders like Amyotrophic Lateral Sclerosis and Frontotemporal Lobe Degeneration. Determining the complete composition of these granules may hold the key to understanding the function and mechanism of assembly of these granules. This work describes multiple approaches taken to identify new protein and mRNA components of P bodies and stress granules. New members of the P body and stress granule proteome reveal a role for these granules in diverse cellular processes including signal transduction, transcription and metabolism. Additionally, a new stress granule resident complex - the CCT complex, was also identified as a novel regulator of granule disassembly. This work also describes the first purification scheme for stress granules and presents a new system for in vitro study of stress granules. Together, the findings shed new light on the composition, function, structure and regulation of P bodies and stress granules in yeast.