Attention can be divided so that multiple objects can be tracked simultaneously as they move among distractors. Although attentional tracking is known to be highly limited, such that most individuals can track only approximately four objects simultaneously, the neurophysiological mechanisms that underlie this capacity limitation have not been established. Here, we provide electrophysiological measures in humans of the initial selection and sustained attention processes that facilitate attentional tracking. Each measure was modulated by the number of objects the subject was tracking and was highly sensitive to each individual's specific tracking capacity. Consequently, these measures provide strong neurophysiological predictors of an individual's attentional tracking capacity. Moreover, by manipulating the difficulty of these two phases of the task, we observe that the limiting factor underlying tracking capacity can flexibly shift between these two attentional mechanisms depending on the requirements of the task.