Since its inception more than 50 years ago, attachment theory has become one of the most influential viewpoints in the behavioral sciences. What have we learned during this period about its fundamental questions? In this paper, we summarize the conclusions of an inquiry into this question involving more than 75 researchers. Each responded to one of nine "fundamental questions" in attachment theory. The questions concerned what constitutes an attachment relationship, how to measure the security of attachment, the nature and functioning of internal working models, stability and change in attachment security, the legacy of early attachment relationships, attachment and culture, responses to separation and loss, how attachment-based interventions work, and how attachment theory informs systems and services for children and families. Their responses revealed important areas of theoretical consensus but also surprising diversity on key questions, and significant areas of remaining inquiry. We discuss central challenges for the future.