At the cellular and molecular levels of operation, neurons and their circuits achieve brain functions by chemical signals, in which the principle agents, neurotransmitters convey the signal from the sending neuron to the receiving neuron. The discovery of each of the chemical families of neurotransmitters ( amino acids, amines and neuropeptides) provide important insights into our understanding of how brains function and the changes in our concepts of the complexities of short term and longer term brain events, and the means by which medications can intervene in brain dysfunctions.