Synaptic inhibition plays a crucial role in regulating neuronal excitability, which is the foundation of nervous system function. This inhibition is largely mediated by the neurotransmitters GABA and glycine that activate Cl-permeable ion channels, which means that the strength of inhibition depends on the Cl gradient across the membrane. In neurons, the Cl gradient is primarily determined by two secondarily-active cation-chloride cotransporters (CCCs), NKCC1 and KCC2. CCC-mediated regulation of the neuronal Cl gradient is critical for healthy brain function, as dysregulation of CCCs has emerged as a key mechanism underlying neurological disorders including epilepsy, neuropathic pain, and autism spectrum disorder. This Review begins with an overview of neuronal chloride transporters before explaining the dependent relationship between these CCCs, Cl regulation, and inhibitory synaptic transmission. We then discuss the evidence for how CCCs can be regulated, including by activity and their protein interactions, which underlie inhibitory synaptic plasticity. For readers who may be interested in conducting experiments on CCCs and neuronal excitability, we have included a section on techniques for estimating and recording intracellular Cl, including their advantages and limitations. While the focus of this Review is on neurons, we also examine how Cl is regulated in glial cells, which in turn regulate neuronal excitability through the tight relationship between this non-neuronal cell type and synapses. Lastly, we discuss the relatively extensive and growing literature on how CCC-mediated neuronal excitability contributes to neurological disorders.