The brain is the organ that collects information from the environment, processes and stores the information, and generates behavior as and when needed. In essence, the brain makes us who we are. For this reason, understanding the biology of brain function is a great challenge and a major goal of modern science. The brain is one of the last great frontiers in science, and the unraveling of its mysteries is comparable in complexity to efforts in space exploration. Therefore, it was not a surprise that the U.S. Congress proclaimed the 1990s the 'Decade of the Brain,' a movement also introduced by several other countries, thereby giving a chance for neuroscientists to focus on this topic and to have better conditions for their research. A fundamental goal of neuroscience is to understand how neurons generate behavior and the pathophysiology of different mental and neurological diseases. This requires, among other things, information about where these neurons are located, how they are connected, and how they communicate with each other in various physiological and pathophysiological conditions. At the end of the nineteenth century, the great neuroanatomist Santiago Ramon y Cajal recognized that neurons are the individual signaling elements of the brain. In this book, we focus on these nerve cells of the brain and the chemical molecules that allow them to talk to each other. The discovery of intercellular communication through endogenous molecules is a milestone in the history of science. It makes the brain a unique organ. Our aim is to describe recent discoveries about the basic operations of the brain and to provide an introduction to the adaptations for specific types of information processing. For example, at a chemical synapse, the presynaptic terminal liberates a transmitter substance that acts on the postsynaptic process. Thus, a synapse converts a presynaptic electrical signal into a chemical signal and back into a postsynaptic electrical signal. Current experimental evidence indicates that this assumption needs to be enlarged. Neurons can communicate in a widespread but still organized mode: transmitters released into the extracellular space may have effects on distant extrasynaptic receptors, exerting a tonic effect. Neurons are able to release their transmitters locally into the extracellular space, where the transmitter diffuses slowly over some distance and influences many other neurons. The notion that the amount of transmitter released by an effective action potential arriving at the nerve terminal is not constant but can be modulated by the chemical environment in the vicinity of the release site is now well accepted, as also the effect of transmitters on postsynaptic sites.