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Hormones modulate various physiological, morphological, and behavioral processes critical to survival and reproduction (Ketterson & Nolan, 1992). Importantly, hormone–behavior associations are bidirectional, whereby hormones may modulate social behavior and social behavior may feed back to influence hormone concentrations. In this chapter, we provide a basic overview of research examining associations between testosterone and human aggression. We begin with a brief review of the literature on the relationship between baseline testosterone concentrations and human aggression. Next, we review evidence that competitive interactions rapidly modulate testosterone concentrations. We then discuss an emerging literature examining the role of competition-induced fluctuations in testosterone in potentiating ongoing and/or future aggressive behavior. In this chapter, we take a comparative approach drawing on both the human and nonhuman literature to provide a sense of the commonalities in the basic neuroendocrine mechanisms underlying complex social behavior.