Neurotransmitters may be classified based on their molecular structure, function, or mode of action. 

Based on chemical and molecular properties, neurotransmitters may be categorized as:

• Amino acids
• Peptides
• Monoamines
• Purines
• Acetylcholine
• Biogenic amines

Based on function, neurotransmitters may be categorized as:

• Excitatory 
• Inhibitory

Based on mode of action, neurotransmitters may be categorized as:

• Direct
• Neuromodulatory