Deep brain stimulation is a remarkable therapy that has mainstreamed electrical stimulation of the brain for the treatment of neurological dysfunction. To appreciate the mechanisms of deep brain stimulation, we need to understand the excitability of neural tissue. Here, we survey the pertinent principles of electrical excitation in the brain. The amount of current delivered and the tissue conductivity together determine the strength and extent of potentials generated by stimulation. The electrode–tissue interface is an important junction where electrical charge carriers in the stimulation hardware are converted to ionic charge carriers in the tissue. Cathodic stimulation tends to depolarize neural elements more easily than anodic stimulation. The current–distance relationship describes how the amount of current needed to excite an axon increases as a function of its distance from the electrode. This relationship also depends on the axon’s diameter because large-diameter axons are excited more easily than small-diameter axons. For a given axon, the strength–duration relationship describes the inverse relationship between threshold current amplitude and pulse duration. Specific stimulation parameters must be considered to avoid stimulation-induced tissue damage. A strong foundation in these principles facilitates understanding of the complex effects of electrical stimulation in the brain.