The regulation of gene expression is a fundamental cellular process that is controlled at multiple levels. Abnormal regulation of gene expression has been directly implicated in the pathogenesis of some diseases of animals and humans and may contribute to the disease process in unrecognized ways in many others. Furthermore, novel treatment strategies for a number of different diseases may hinge upon our ability to exploit mechanisms that normally alter the expression of endogenous genes. While the study of gene regulation has traditionally focused on transcription as a major regulator of gene expression, it has recently become apparent that the post-transcriptional control of gene expression may play an equally important role. In particular, rapid, context-specific regulation of the stability of mRNA transcripts encoding highly active proteins, such as cytokines, growth factors, oncogenes and cell-cycle regulators, appears to play a key role in the control of these molecules and the processes they mediate. Many of the known regulatory pathways for mRNA stability involve proteins that interact with specific AU-rich elements in the 3'-untranslated region of the transcript. This review will address some important aspects of the normal regulation of mRNA stability and known or potential contributions of RNA stability regulation to health and disease.