How are synapses made? This question is one of the most important issues in neurobiology today and has been the subject of intense study in recent years. This review focuses on the mechanisms of presynaptic terminal formation in the mammalian central nervous system. Building a synapse requires stabilization of contacts between axons and dendrites and formation of synaptic subcellular structures. Here, we discuss what determines where and when synapses form, how components of the nascent presynaptic terminal accumulate at the site of synapse formation, and whether assembly occurs via an ordered process dependent on a master organizer. Understanding synapse formation in the central nervous system is relevant for understanding and treating brain diseases as diverse as autism, epilepsy, anxiety disorders, brain injury, and Alzheimer's disease.