The brain contains a greater diversity and abundance of microRNAs than any other organ in the body. MicroRNAs stay busy long after they’ve coordinated brain development, but doing what? In the brain, microRNAs serve two somewhat contradictory roles: enforcing the stable patterns of genes that define mature circuits while at the same time conferring the same cells with the flexibility to adapt to changing information. This chapter begins with the basic principles of brain function and some early discoveries on microRNAs in the brain. It explores how the microRNA system influences learning, memory and emotions. It also looks at the evidence that a rich and diverse pool of microRNAs contributed to evolved intelligence. It explains the molecular cues that signpost microRNAs to go to synapses, and how the amount of microRNA activity is linked to the incoming strength of signals. It then looks in depth at some specific microRNAs and their targets and how their competing actions adjust the strength of contacts between neurones. Finally, it looks at how genetic variation and erroneous amounts of certain microRNAs may contribute to risk of neuromuscular and psychiatric disease.