||The biological question that interests me is: How do we make functional nervous and vascular systems? One of the first steps in this process is guiding neurons with their axons and dendrites, and vascular endothelial cells, toward the right places at the right times. To address the question of guidance, I have looked at Netrin, a guidance molecule known to be important in axonal guidance, and its role in guiding dendrites and vascular endothelial cells in vivo during zebrafish embryonic development. The dendritic guidance work has been done in the octavolateralis efferent system, which gives efferent input to the lateral line and ear and has pronounced dendrites that cross the midline. For the vascular work, I have concentrated on the development of the trunk vasculature and particularly on the formation of the parachordal vessel, a venous vessel that extends along the horizontal myoseptum and interconnects venous intersegmental vessels. Chapter 1 is an introductory chapter in which I present the major questions I am trying to answer and the appropriate background on guidance, the octavolateralis efferent system, the vascular system, and Netrin signaling. Chapter 2 is a peer-reviewed paper published in "The Journal of Neuroscience" in which I show that Netrin is important for the contralateral dendrites of the octavolateralis efferent neurons as they extend toward the midline. Chapter 3 is a collaborative peer-reviewed paper published in the journal Science giving evidence that Netrin promotes angiogenesis, or sprouting of blood vessels from preexisting vessels, in vivo and in vitro. I contributed all of the zebrafish results. Chapter 4 is written in the format of a peer-reviewed paper and shows the latest work I have done in finding the mechanism by which Netrin promotes parachordal vessel (PAV) formation. Finally, Chapter 5 (Conclusions) discusses the findings of this work and future directions.