How do we understand and engage with the purposeful, emotional and mental activities of other people and how does this knowledge develop? What can recent work on mirror neurons in monkeys and human beings teach us about how the brain supports social understanding? According to Intentional Relations Theory (Barresi and Moore 1996), the understanding of the self-other equivalence requires concurrent knowledge of mind from both a first- and a third-person point of view and that any mental concept must directly match and link these two ways of knowing it. In this paper, it is argued that Intentional Relations Theory is consistent with and can help interpret recent neurophysiological findings on “mirror neurons” that fire equivalently for intentional relations (i.e., object-directed actions, emotions, and mental activities) of self and other.