Central to mRNA metabolism is the poly(A)-tail, a stretch of adenosine nucleotides at the mRNA 3' end. In this dissertation, I investigate the role of the tail in the dynamics of mRNA decay, and describe the predominant mechanisms of decay for thousands of mammalian mRNAs. Next, I examine the effects of microRNAs, which influence mRNA decay and perturb tail length dynamics. Finally, I describe a physiological context in which the tail helps to control translation: neurons of the mouse brain. mRNA decay is tightly regulated in eukaryotes, determining the steady-state abundances and rates of accumulation of mRNAs. Despite this central role, the dynamics of decay have been described for only a handful of mRNAs. We determine these dynamics for thousands of endogenous mRNAs. Nascent mRNAs have reproducible and heterogeneous tail lengths just after they escape the nucleus.