Numerous in vitro assays and in vivo perturbation studies have led to a model of neural development in which selective fasciculation helps to define accurate axonal projections. Genetic analysis in vivo confirms the hypothesis that axonal fasciculation and defasciculation are controlled by adhesion mechanisms, but also suggests that, in many cases, adhesion and guidance are separable phenomena. In addition, receptors that control the level of tyrosine phosphorylation may play an important role in fasciculation, suggesting that complex intracellular pathways lie just beneath the surface.