Few psychometric studies have confirmed the factor structure of the Attachment Style Questionnaire (ASQ) (Feeney, Noller, & Hanrahan, 1994), a widely used self-report attachment measure. Moreover, no study has formally investigated the factor structure of the ASQ’s short form (ASQ-SF) proposed by Alexander, Feeney, Hohaus, and Noller (2001). The aim of the present study was to validate the factor structures of the ASQ and ASQ-SF, and to identify the more parsimonious measure. In two studies, a nested factor model provided the best fit, the ASQ-SF was the more parsimonious measure, and results were consistent across age and gender groups. Theoretical and clinical implications are discussed.