The importance of microRNAs during development is well recognized, but a number of studies indicate that their function extends to differentiated cells as well. Metabolic control and neural plasticity are dynamic processes in adult organisms and represent good examples where miRNA-mediated regulation affects the function of differentiated cells without changing their fate. We are summarizing the current understanding of how microRNAs contribute to post-transcriptional control of gene expression in these processes and, where possible, highlight in which mode of action--the "guardian" or the "tuning mode"--they are employed by the cell.