Astrocytes require perineuronal nets (PNNs) to maintain synaptic homeostasis in mice. PNNs are dense extracellular matrices that cover the cell bodies of fast-spiking inhibitory neurons and stabilize synapses by inhibiting synaptic plasticity. In the adult mouse somatosensory cortex, PNNs have holes that allow synaptic terminals of fast-spiking interneurons to access the extracellular space. These holes contain synapses and astrocytic processes expressing Kir4.1, glutamate, and GABA transporters, forming tripartite synapses. PNN degradation leads to expanded astrocytic coverage of neuronal somata without altering axon terminals. The loss of PNNs impairs astrocytic uptake of synaptically released glutamate and potassium, causing glutamate to spill into the extrasynaptic space. PNNs and astrocytes cooperate to contain synaptically released signals, and their combined action is altered in mouse models of Alzheimer's disease and epilepsy, where PNNs are disrupted. Excitatory synapses associate with astrocytic processes, allowing astrocytes to sense and modulate synaptic activity. Tripartite synapses enable astrocytic processes to remove neurotransmitters and ions released during synaptic activity, particularly ensuring that glutamate does not spill out of synapses and activate extrasynaptic receptors, causing excitotoxicity. The extracellular matrix (ECM) surrounding neurons and astrocytes includes proteoglycans, glycoproteins, and polysaccharides, such as hyaluronan, chondroitin sulfate proteoglycans, aggrecan, brevican, versican, neurocan, tenascin, and link proteins. Some ECM components interact with synaptic receptors and ion channels, affecting synaptic vesicle release, dendritic spine morphology, and synaptic structural integrity. The ECM also alters ion diffusion in the extracellular space. A highly condensed form of ECM forms a corset-like structure known as PNNs, mainly around parvalbumin (PV) expressing inhibitory neurons. PNNs are easily recognized by the binding of wisteria floribunda agglutinin (WFA) and encapsulate the cell soma, dendrites, and axon initial segment. PNNs stabilize synapses and restrict synaptic plasticity, particularly in pathways with developmental activity-dependent plasticity, such as the visual system. However, whether PNNs interact structurally and functionally with astrocytes at tripartite synapses has not been studied. The study shows that synapses onto PNN-expressing fast-spiking neurons (FSNs) occupy small perforations or holes within the PNNs, where excitatory and inhibitory synapses co-mingle with astrocytic processes, forming tripartite synapses. PNN disruption impairs astrocytic uptake of synaptically released glutamate and potassium, causingAstrocytes require perineuronal nets (PNNs) to maintain synaptic homeostasis in mice. PNNs are dense extracellular matrices that cover the cell bodies of fast-spiking inhibitory neurons and stabilize synapses by inhibiting synaptic plasticity. In the adult mouse somatosensory cortex, PNNs have holes that allow synaptic terminals of fast-spiking interneurons to access the extracellular space. These holes contain synapses and astrocytic processes expressing Kir4.1, glutamate, and GABA transporters, forming tripartite synapses. PNN degradation leads to expanded astrocytic coverage of neuronal somata without altering axon terminals. The loss of PNNs impairs astrocytic uptake of synaptically released glutamate and potassium, causing glutamate to spill into the extrasynaptic space. PNNs and astrocytes cooperate to contain synaptically released signals, and their combined action is altered in mouse models of Alzheimer's disease and epilepsy, where PNNs are disrupted. Excitatory synapses associate with astrocytic processes, allowing astrocytes to sense and modulate synaptic activity. Tripartite synapses enable astrocytic processes to remove neurotransmitters and ions released during synaptic activity, particularly ensuring that glutamate does not spill out of synapses and activate extrasynaptic receptors, causing excitotoxicity. The extracellular matrix (ECM) surrounding neurons and astrocytes includes proteoglycans, glycoproteins, and polysaccharides, such as hyaluronan, chondroitin sulfate proteoglycans, aggrecan, brevican, versican, neurocan, tenascin, and link proteins. Some ECM components interact with synaptic receptors and ion channels, affecting synaptic vesicle release, dendritic spine morphology, and synaptic structural integrity. The ECM also alters ion diffusion in the extracellular space. A highly condensed form of ECM forms a corset-like structure known as PNNs, mainly around parvalbumin (PV) expressing inhibitory neurons. PNNs are easily recognized by the binding of wisteria floribunda agglutinin (WFA) and encapsulate the cell soma, dendrites, and axon initial segment. PNNs stabilize synapses and restrict synaptic plasticity, particularly in pathways with developmental activity-dependent plasticity, such as the visual system. However, whether PNNs interact structurally and functionally with astrocytes at tripartite synapses has not been studied. The study shows that synapses onto PNN-expressing fast-spiking neurons (FSNs) occupy small perforations or holes within the PNNs, where excitatory and inhibitory synapses co-mingle with astrocytic processes, forming tripartite synapses. PNN disruption impairs astrocytic uptake of synaptically released glutamate and potassium, causing