Understanding is at the heart of intimate relationships. It is unclear, however, whether understanding-partners' subjective feeling that they understand each other-or knowledge-partners' accurate knowledge of each other-is more important for relationship well-being. The present article pits these two types of understanding against each other and investigates their effects on relationship well-being. In a prospective study among 199 newlywed couples, partners' self-reported and perceived understanding and their knowledge in different domains were assessed. Understanding was independent of knowledge. Self-reported and perceived understanding predicted relationship well-being but neither type of knowledge did. Thus, subjectively feeling that one understands and is understood by one's partner appears to be more important to relationship well-being than actually knowing and being known by one's partner. © 2009 by the Society for Personality and Social Psychology, Inc.