Our long-term goal is to understand how large systems of nerve cells operate. We have focused on two sensory systems: the retina and the olfactory bulb. Visual perception derives from the action potentials transmitted to the brain through the two optic nerves, each containing about one million fibers that originate in retinal ganglion cells. How is the information about our visual environment encoded at this stage of the nervous system? And how is that code generated by the circuits within the retina? These studies involve recording the neural activity of many retinal ganglion cells and analyzing its relationship to the visual stimulus. The underlying circuit mechanisms are probed with pharmacology, electrical stimulation, and transgenic modifications of the retina. For the olfactory system the space of stimuli consists of several 100,000 volatile chemicals, which, unlike visual images, are not easily decomposed into simple basic constituents. Correspondingly, the nervous system employs about 1000 different types of receptors to sample these stimuli, whereas our visual system has at most 4. In recent work we have focused on delineating how this large population of receptors samples the vast space of stimuli, and how the resulting neural information is organized at the first stage of processing, the olfactory bulb.