The computational power of the human brain derives from its neuronal wiring. The brain has about 100 billion neurons, and each neuron connects with hundreds of other neurons via thousands of synapses. Thus, the brain has over 100 trillion (10**14) synapses - this staggering number of connections, together with the precision of wiring between neurons, enables the complexity of brain processing. Neuronal circuits and networks are the engine of the brain, for they transform simple inputs into complex outputs. Novel technologies are transforming the analysis of brain circuits. Research in our laboratory demonstrates how specific circuits of the cerebral cortex mediate feature selectivity in vision, temporal coding by neuronal populations, and internal states of the brain. The logic of these circuits reveals fundamental principles of information processing underlying behavior and cognition, and lays the groundwork for rich computational, theoretical and mechanistic analyses of normal and abnormal brain function.