Neurotransmitters should fulfil the following main three criteria; 1) they should be localized in synaptic terminals, 2) they should have certain actions on neurons, 3) they should be released from the terminals after nerve stimulation. In addition to the above three, recent studies have revealed that neurotransmitters should have their own receptors. Moreover, recent immunohistochemical studies have revealed certain neurons to have two or more neurotransmitter candidates; one classic and the other non-classic neurotransmitter candidate. These progresses in the field of neurotransmitter science were briefly reviewed. As an extension of the basic studies mentioned above, clinical studies of measurements of neurotransmitter markers in brains of neurodegenerative disorders are now actively performed. Based on my own studies in the same line, it is possible to summarize the fundamental patterns of neurotransmitter alterations in neurodegenerative disorders as follows: 1) they decrease as a result of degeneration of neuron groups (the most popular pattern); 2) they decrease in spite of the morphologically normal appearance of neurons ("biochemical non-functioning"); 3) they increase in the projecting regions of certain neurons which escaped degeneration. This may be a result of simple condensation of normal nerve terminals or real increase based on the compensatory mechanisms.