Volume transmission is a neuromodulatory signaling mechanism distinct from classical synaptic transmission. It does not rely on synaptic contacts and is the primary mode of action for monoamines and neuropeptides in the brain. Unlike synaptic transmission, which is precise and occurs within milliseconds, volume transmission is less precise and involves broader spatiotemporal scales. This review outlines five domains in which volume transmission systems differ from synaptic transmission and from each other: (1) innervation patterns and firing properties, (2) transmitter synthesis and loading into vesicles, (3) architecture and distribution of release sites, (4) transmitter diffusion, degradation, and reuptake, and (5) receptor types and their positioning. The review discusses these domains for dopamine, a well-studied monoamine, and compares them with those of norepinephrine and serotonin. It also includes assessments of neuropeptide signaling and central acetylcholine transmission. Dopamine is synthesized from tyrosine and loaded into vesicles via VMAT2. Its release is mediated by SNARE proteins and extracellular Ca²+ and involves active zone-like protein architecture. Dopamine release sites are sparse, and release is highly probable. Diffusion and reuptake determine local dopamine levels, with reuptake shaping the signal. Receptor activation depends on the relative positioning of dopamine receptors to release sites, though these spatial relationships remain unknown. Norepinephrine and serotonin are also released from vesicles and share some similarities with dopamine in terms of release mechanisms. However, their release is less dependent on Ca²+ channels and is more resistant to CaV1 blockade. Their release sites are also sparse, and their signaling is influenced by diffusion and reuptake. Acetylcholine acts as a volume transmitter in the central nervous system, with signaling mediated by GPCRs. Cholinergic interneurons in the striatum activate metabotropic muscarinic receptors and trigger dopamine release. Acetylcholine is rapidly degraded by acetylcholine esterase, limiting its spread. Neuropeptides are released from LDCVs and act through GPCRs. They are produced in the soma and transported via long-range pathways. Neuropeptide release requires strong stimulation and depends on Ca²+ and voltage-gated Ca²+ channels. Neuropeptide receptors are mostly metabotropic GPCRs and are expressed throughout the brain. In conclusion, volume transmission mechanisms are heterogeneous, and detailed knowledge of release site structure, function, and receptor positioning is essential for understanding neuromodulatory systems. Current research is focused on developing tools for systematic assessment of neuromodulatory transmission mechanisms, including fluorescent sensors. These tools will help bridge knowledge gaps between mechanistic features of volume transmission and in vivo neuromodulatory dynamics. A molecular and cellular understanding of volume transmission will help define how these systems regulate brain function and how they can be targeted for treating disease.Volume transmission is a neuromodulatory signaling mechanism distinct from classical synaptic transmission. It does not rely on synaptic contacts and is the primary mode of action for monoamines and neuropeptides in the brain. Unlike synaptic transmission, which is precise and occurs within milliseconds, volume transmission is less precise and involves broader spatiotemporal scales. This review outlines five domains in which volume transmission systems differ from synaptic transmission and from each other: (1) innervation patterns and firing properties, (2) transmitter synthesis and loading into vesicles, (3) architecture and distribution of release sites, (4) transmitter diffusion, degradation, and reuptake, and (5) receptor types and their positioning. The review discusses these domains for dopamine, a well-studied monoamine, and compares them with those of norepinephrine and serotonin. It also includes assessments of neuropeptide signaling and central acetylcholine transmission. Dopamine is synthesized from tyrosine and loaded into vesicles via VMAT2. Its release is mediated by SNARE proteins and extracellular Ca²+ and involves active zone-like protein architecture. Dopamine release sites are sparse, and release is highly probable. Diffusion and reuptake determine local dopamine levels, with reuptake shaping the signal. Receptor activation depends on the relative positioning of dopamine receptors to release sites, though these spatial relationships remain unknown. Norepinephrine and serotonin are also released from vesicles and share some similarities with dopamine in terms of release mechanisms. However, their release is less dependent on Ca²+ channels and is more resistant to CaV1 blockade. Their release sites are also sparse, and their signaling is influenced by diffusion and reuptake. Acetylcholine acts as a volume transmitter in the central nervous system, with signaling mediated by GPCRs. Cholinergic interneurons in the striatum activate metabotropic muscarinic receptors and trigger dopamine release. Acetylcholine is rapidly degraded by acetylcholine esterase, limiting its spread. Neuropeptides are released from LDCVs and act through GPCRs. They are produced in the soma and transported via long-range pathways. Neuropeptide release requires strong stimulation and depends on Ca²+ and voltage-gated Ca²+ channels. Neuropeptide receptors are mostly metabotropic GPCRs and are expressed throughout the brain. In conclusion, volume transmission mechanisms are heterogeneous, and detailed knowledge of release site structure, function, and receptor positioning is essential for understanding neuromodulatory systems. Current research is focused on developing tools for systematic assessment of neuromodulatory transmission mechanisms, including fluorescent sensors. These tools will help bridge knowledge gaps between mechanistic features of volume transmission and in vivo neuromodulatory dynamics. A molecular and cellular understanding of volume transmission will help define how these systems regulate brain function and how they can be targeted for treating disease.