For decades, social scientists have observed that people greatly desire a partner who is similar to themselves. Less is known, however, about whether particular similarity domains (e.g., music preferences) may uniquely influence relationship formation. We address this gap by examining people’s preferences for 18 similarity domains in three types of relationships: friendships, casual/short-term, and long-term. The most important similarity domains, across the three relationship types, were political views, career goals, food preferences, travel desires, and music preferences. General similarity was most important in long-term rather than in friendships and casual/short-term relationships, with the latter two relationship types not differing from one another. This pattern emerged for all similarity domains with four exceptions: preferences for books, video games, computer brands, and cell phone brands. No sex differences emerged in similarity domains except in preferences in video games and brands of cell phones and computers. Men rated these domains to be more important than did women. All three of these differences were of relatively small effect size. We tie this work into the larger body of research on similarity and preferences for partner traits.