This chapter focuses on the syndromes that are associated with antibodies that target proteins of the inhibitory synapses. Two antibodies are directed against intracellular presynaptic proteins, including glutamic acid decarboxylase (GAD), a key enzyme in the synthesis of GABA, and amphiphysin, which is involved in the presynaptic reuptake of neurotransmitters. Both antibodies are associated with stiff-person syndrome (SPS), which results in rigidity in proximal muscles of legs, abdomen, and lower back, impaired gait, muscles spasms, exaggerated startle responses to acoustic or tactile stimuli, and anxiety and task-specific phobias. Three antibodies are directed against cell surface receptors, including GABAaR and GABAbR, and the glycine receptor (GlyR). Encephalitis with prominent seizures is the common presentation of patients with antibodies against GABAaR or GABAbR, whereas antibodies against GlyR associate with an SPS variant named progressive encephalomyelitis with rigidity and myoclonus (PERM). In addition, this chapter includes the antibodies against dipeptidyl-peptidase-like protein 6 (DPPX), an auxiliary subunit of the Kv4.2 potassium channels that is not restricted to inhibitory synapses, but patients with this disorder frequently show CNS hyperexcitability and sometimes clinical features similar to PERM.