Roukes and Shepard propose next-generation, nanofabricated multielectrode arrays for recording and stimulating neural activity on a massive scale. The team will develop ultra-thin silicon probes, which will be mass-produced via partnerships with commercial grade semiconductor foundries, and will be widely disseminated to the neuroscience community. In addition to electrical recording and stimulating electrodes, arrays will be fabricated for multimodal experimental capabilities, including electrochemical sensing for neurotransmitters such as glutamate and dopamine, and nanoscale optical waveguides for optogenetic stimulation. Planar arrays of implantable shanks will be scaled to over 8,000 contact sites, establishing 3D systems with high bandwidth. The new multifunctional arrays will be tested in neuroscience research labs working on such wide-ranging topics as Parkinson's disease, the role of sleep in memory consolidation, and the function of neural circuits in different sensory systems.