A mature neuron is typically polarized with a single long axon and several dendrites. After the birth and differentiation of a neuron, a neuron breaks its previous symmetry and establishes an axon and dendrites. Neuronal polarization occurs when one of the multiple immature neurites emerging from the cell body elongates rapidly. This neurite becomes the axon, whereas the remaining immature neurites become dendrites. What are the molecular mechanisms specifying the axon in the initial events? Here we provide an overview of recent progress into the study of axon formation.