For better or worse, most of us find ourselves surrounded by others whose goals and interests are not perfectly aligned with our own. And so each day, most of us face one or more versions of the same basic question: How hard should I push? When people assert themselves forcefully, they may get their way in some instrumental fashion, but they may fail to get along with their counterparts. When people forego pursuing their interests, they may get along with others, but their acquiescence may mean a failure to get their way. In this talk, I examine this balancing act, reviewing research on the perceptions and sources of interpersonal assertiveness. I discuss work that suggests perceivers often see actors as pushing too hard or failing to push hard enough, suggesting that the balancing act of assertiveness is a challenge for many people. I also present an account of the sources of interpersonal assertiveness, considering why some people in interpersonal conflicts are more likely to give in whereas others press aggressively to get their way. Past work has tended to focus on motivational factors that shape assertiveness, yielding abundant evidence that motivations matter. Here, I focus on expectancies—idiosyncratic predictions people make about the social and instrumental consequences of assertive behavior—as a source of behavior. Across several studies, I unpack the nature of assertiveness expectancies and find that individual differences in these expectancies predict behavioral assertiveness. These results suggest that expectancies deserve an important place alongside motivations in our psychological models and interventions.