Within the central and peripheral nervous systems of animals, including man, inhibition is crucial to counterbalance excitatory neurotransmission, which is predominantly mediated by glutamate and its receptors. Although, particularly in brain, much of this inhibition is provided by classical post-synaptic GABA_A receptors, many other proteins and mechanisms regulate excitation. These exist both to "fine tune" neurotransmission and to prevent overexcitation that could lead to conditions such as epilepsy and excitotoxicity, which can result in cell death. This book reviews aspects of GABA_A receptor function, as well as the properties of a variety of other important inhibitory proteins, such as GABA_C receptors, G-protein coupled receptors (specifically, GABA_B receptors, metabotropic glutamate receptors and neuropeptide receptors), glycine receptors, GABA transporters and potassium channels. In addition, the consequences of mutations that disrupt the regulation of excitatory neurotransmission, and efforts to target the GABAergic system for therapeutic benefit, are discussed.