Over the past decade, microRNA molecules have emerged as critical regulators in the expression and function of animal genomes. This review discusses the relationship between microRNA-mediated regulation and the robustness of biochemical networks that contain microRNAs. Most biochemical networks are robust; they are relatively insensitive to the precise values of reaction constants and concentrations of molecules acting within the network. MicroRNAs involved in network robustness may appear to be nonessential under favourable uniform conditions used in conventional laboratory experiments. However, the function of these molecules can be revealed under environmental and genetic perturbations. Recent advances have revealed unexpected features of microRNA organization in networks that help explain their promotion of robustness.