Neurotransmitter is a chemical substance or a chemical messenger within brain, i.e. secreted by neurons and that allow communication between nerve cells to produce physiological response. Neurotransmitters are classified on the basis of their chemical composition. Various stimuli physical, chemical or electrical activate neurons from their resting state and cause them to produce nerve impulses. These nerve impulses cause the release of neurotransmitters in the synaptic cleft. Neurotransmitters are of two types, excitatory, e.g. acetylcholine, serotonin etc. and inhibitory, e.g. GABA, glycine, etc. The degradation or reuptake of neurotransmitters is necessary to limit the duration of stimulation or inhibition of the post synaptic membrane. The principle catecholamines, norepinephrine, epinephrine and dopamine, are the neurotransmitters synthesized from tyrosine. This chapter describes the various classes of neurotransmitters, their synthesis, release mechanism, storage and function.