Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons is critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both these classes of synapses. In addition, electrical and chemical synapses can mutually co-regulate each other's formation. Electrical activity, generated spontaneously by the nervous system, or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.