The establishment of sensory-organ innervation is crucial for behaviour, social interactions and survival. Neuronal arborisation is dictated by signals that guide neurites to their appropriate targets in combination with the spatial distribution of the target cells. To date, most emphasis has been placed on the genetic bases of neuronal-type specification and organ formation during embryonic development and, consequently, there is a serious gap in our knowledge about the long-term dynamics of organ innervation. To advance towards this goal, we will need to elucidate whether and how neuronal activity and sensory organ architecture can modulate neural arborisation. Vertebrate sensory organs represent an excellent model to study the establishment and maintenance of organ innervation. To investigate these processes, I have chosen the zebrafish mechanosensory “lateral line” as a model system. In general, planar polarized sensory hair cells in each unit of the lateral line, called neuromast, receive innervation by two afferent neurons. Hair cells can regenerate and re-innervate very rapidly, providing a simple model of sensory neurons arborisation whose dynamics can be visualized in vivo over long periods and under normal or altered physiological conditions. To investigate the mechanisms that govern the acquisition and maintenance of hair-cell innervation, I will conduct a systematic and comprehensive study of the processes that underlie the elaboration and remodelling of sensory-neuronal architecture in the lateral-line organ. I will perform high-resolution live imaging combined with sophisticated techniques to permit spatio-temporal control of protein expression. The findings derived from these studies shall reveal the mechanisms by which animals maintain sensory abilities throughout their entire lives.