The nervous systems of invertebrates and vertebrates consist of neuronal networks of varying complexity, and the elucidation of the organization of these networks is essential if we are to understand neural function. Up until the mid-19th Century gross dissection was the primary tool available to scientists to study the nervous system. The development of neurohistological techniques, electrical stimulation, and observation of neural function in humans and animals following injury added rapidly to our understanding of the nervous system during the following century. Over the last 3 decades investigators seeking to unravel the complexities of neural circuits have made use of analytical methods based upon the biological properties of neurons, including orthograde and retrograde axonal transport of tracer substances, the expression of particular genes and gene products that can be assessed with immunocytochemical or in situ methods, and the imaging of the utilization of oxygen or glucose by active populations of neurons. Advances in neuroscience have led to an enormous expansion in our knowledge of normal neural functioning and how that function is altered by injury or disease. Modern studies of neuronal organization have been at the center of our increased understanding of how the brain works.