Sensory neurons utilize complex, topical networks of dendritic processes to detect external stimuli. Models that seek to explain the creation of these elaborate structures must include mechanisms that control the key events of branch initiation, elongation and termination. These features are universally observed in both vertebrate and invertebrate systems and are therefore likely governed by evolutionarily conserved components. The simple model organism, C. elegans, displays a single pair of PVD nociceptive neurons that envelop the animal with a net-like array of dendritic branches. We used time-lapse imaging to show that the discrete topical region occupied by each branch is defined by a contact-dependent mechanism in which sister dendrites (i.e., dendrites from the same neuron) repel each other to stop outgrowth. Self-avoidance is also observed in mammals and insects but the molecular underpinning of this fundamental patterning event is poorly understood. Our work has revealed a novel mechanism in which the diffusible cue UNC-6/Netrin is captured at the tips of PVD dendrites to mediate self-avoidance in a pathway involving the receptors UNC-40/DCC and UNC-5. This discovery is significant because it describes the first example of a role for these highly conserved proteins in dendrite self-avoidance. Now, we have extended these findings to identify multiple downstream components of the UNC-6/Netrin self-avoidance pathway. On the basis of these new results, we propose that UNC-6/Netrin triggers actin filament growth at the tips of contacting sister dendrites to engage a non-muscle myosin motor that drives retraction. Experiments described in Specific Aim 1 exploit the novel application of TIRF microscopy to a living organism to test this model. These studies are significant because little is known of how signals at the cell membrane trigger dendrite withdrawal during self-avoidance. Our work has uncovered a key role for a conserved membrane protein, tomoregulin, in self-avoidance. Specific Aim 2 will define the mechanism of this effect and determine if tomoregulin is necessary for other known short-range UNC- 6/Netrin signaling events. Aim 2 is significant because it addresses the fundamental question of how the UNC-6/Netrin pathway has been uniquely adapted for contact-dependent self-avoidance. To address the mechanism of dendritic outgrowth, we exploited powerful cell-specific profiling methods to identify targets of a conserved LIM-homeodomain transcription factor, MEC-3, that is required for PVD branching. Specific Aim 3 will test a model, based on these results, that dendritic branches are stabilized by interaction with claudin-like proteins and other specific cell-surface components in the adjacent epidermis. These experiments are important because sensory neuron outgrowth is typically executed in close contact with epidermal tissue but the intercellular mechanisms that pattern dendritic architecture in this location are poorly defined. This work in C. elegans is expected to identify key determinants that also specify dendritic architecture in the human nervous system.