Social network research has tended to shun psychological explanations of structural patterns. But this structuralist dismissal of the individual is countered by a research program that examines how individuals differ in the management of social relationships and situations. Self-monitoring theory develops the concept of the negotiated self that acts out different roles for different audiences. In effect, the high self-monitor is homo sociologicus, earnestly attentive to the requirements of social life, paying attention to the norms and rituals required for successful role performance, eager to gain social status. The low self-monitor, by contrast, is homo psychologicus, striving to be true to the inner self, to act in accordance with deeply-held principles and values rather than giving in to the varying social pressures that each situation presents. Paralleling, therefore, the distinction between oversocialized and undersocialized conceptions of human actors, self-monitoring theory offers prototypical personality types, with the difference that these types are conceptualized as engaged with social situations. This is what makes self-monitoring different from other personality theories and, at the same time, highly relevant for understanding social network interactions. And from this theory there derive numerous surprising results of relevance for social network research.