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A neurotransmitter is one of a class of chemical substances that carry messages between neurons. Typically, a sending neuron releases small amounts of a neurotransmitter, and this activates receptors on the receiving neuron. Receptor activation then initiates a series of chemical changes in the receiving neuron, and if enough receptors are activated, the receiving neuron may itself become active and send the message along.

A variety of neurotransmitters have been identified, including acetylcholine, dopamine, serotonin, and norepinephrine. Typically, neuronal receptors are specialized to only respond to one type of neurotransmitter. This allows for a high degree of specialization in how messages are transmitted between neurons: one neuron may respond strongly to release of a particular neurotransmitter while its neighbor may be relatively insensitive.

Neurotransmitter imbalances have been implicated in several diseases such as Alzheimer's disease and Parkinson's disease, and in a variety of psychiatric illnesses such as schizophrenia and depression. Many drugs work by altering the level of specific neurotransmitters in the brain (see, e.g., cholinesterase inhibitors).