Gene expression is a combination of many processes, including transcription, pre-mRNA processing, nucleocytoplasmic transport of mRNA, translation, mRNA decay, and protein modification and decay. Many changes in the programs of gene expression occur during development, differentiation, and aging. These alterations are reflected at both the mRNA and protein levels. While altered gene expression at the levels of transcription and protein turnover has been appreciated for some time, mRNA decay is now emerging as an important control point and a major contributor to gene expression as well. Continuing identification of the protein factors and cofactors, and mRNA instability elements, responsible for mRNA decay are allowing us to build a comprehensive picture of the highly orchestrated processes involved in mRNA decay and its regulation. Here, I survey mRNA decay processes in eukaryotes and describe some molecular mechanisms that alter the decay rates of specific mRNAs, leading to major changes in gene expression during development and aging.