MicroRNAs (miRNAs) are ~22 nt RNAs that coordinate vast regulatory networks in animals and thereby influence myriad processes. This Review examines evidence that miRNAs have continuous roles in adults in ways that are separable from developmental control. Adult-specific activities for miRNAs have been described in various stem cell populations, in the context of neural function and cardiovascular biology, in metabolism and ageing, and during cancer. In addition to reviewing recent results, we also discuss methods for studying miRNA activities specifically in adults and evaluate their relative strengths and weaknesses. A fuller understanding of continuous functions of miRNAs in adults has bearing on efforts and opportunities to manipulate miRNAs for therapeutic purposes.