The brain can be studied by methods ranging from genetics and molecular biology to behavioral testing and non-invasive imaging in healthy and neurologically compromised humans. Such studies have created ever-increasing knowledge about the organization of the nervous system. Many of the most notable successes in modern neuroscience have come from understanding nerve cells as the structural and functional units of the nervous system. The organization of neurons and glia into neural circuits provides the substrate for processing of sensory, motor, and cognitive information. The physiological characterization of the neurons and glia in such circuits, as well as more detailed analysis of the electrical properties of individual neurons, recorded with electrodes placed near or inserted inside the cell, provide insight into the dynamic nature of information processing, sensory maps, and the overall function of neural systems. Minimally invasive and non-invasive imaging methods can be used in humans with or without brain damage or disease. These methods can help integrate observations from animal models and human post-mortem analysis with structure and function in the living human brain. Among the goals that remain are understanding how basic molecular genetic phenomena are linked to cellular, circuit, and system functions; understanding how these processes go awry in neurological and psychiatric diseases; and understanding the especially complex functions of the brain that make us human.