Partners in close relationships can be both accurate and biased in their perceptions of each other. Moreover, sometimes a bias can lead to accuracy. The authors describe a paradigm for the simultaneous measurement of accuracy and bias in 2-person relationships. One prevalent bias in close relationships is assumed similarity: Does the person think that his or her partner sees the world as he or she does? In a study of 238 dating and married heterosexual couples, the authors found evidence for both bias and accuracy: the bias effects were considerably stronger, especially when the measure was linked to the relationship. They found little or no evidence for gender differences in accuracy and bias.