This chapter reviews that cholinergic interneurons exhibit multiple discharge patterns including two forms of rhythmic discharge – single spiking and bursting – as well as irregular firing. The mechanism that prevents the existence of a stable resting potential in these cells, and thus guarantees that they will always discharge, relies on the presence of robust persistent sodium current that repeatedly brings the cells to action potential threshold. Thus, even in the absence of synaptic input the cholinergic interneurons fire autonomously. A host of high-voltage activated calcium currents, expressed by the cells, which are selectively coupled to specific calcium-activated potassium currents, participates in selecting among the various firing patterns. While the basic mechanisms of discharge are intrinsic to the cholinergic interneurons, synaptic input can exert an important influence on the discharge of these neurons. Weak ionotropic inputs, resulting from glutamatergic inputs from the intralaminar nuclei or from the cortex, or from gamma-aminobutyric-acid (GABAergic) inputs from striatal neurons, can nudge the timing of individual action potentials. The extended and dense axonal arborization of the network of cholinergic interneurons, combined with their shared pause responses in primates, led to the proposal that these neurons act in concert to modulate activity of the whole striatal network. Tonically active neuronsTANs have been shown to discharge and pause synchronously. Future work needs to determine to what extent this synchrony is controlled by the firing pattern of the cholinergic interneurons.