The role of small RNAs as key regulators of mRNA turnover and translation has been well established. Recent advances indicate that the small RNAs termed microRNAs play important roles in animal development and physiology. Cellular activities such as proliferation, morphogenesis, apoptosis and differentiation are regulated by microRNAs. The expression of various genes are regulated by microRNAs, and several microRNAs act in reciprocal negative feedback loops with protein factors to control cell fate decisions that are triggered by signal transduction activity. These observations implicate small RNAs as important mediators of gene regulation in response to cell–cell signaling. The mechanism by which microRNAs silence gene expression is post-transcriptional, possibly influencing the stability, compartmentalization and translation of mRNAs. This mechanism is an efficient means to regulate production of a diverse range of proteins.