The authors describe “solitude theory,” a “lonely” branch of psychological research and theory, at present cut off from reigning contemporary viewpoints, in particular attachment theory. After presenting the history and present state of solitude theory, the authors argue that the seeming contradictions between solitude theory and attachment-centered theories are based on a certain definition of solitude-seeking behaviors, one that emphasizes “separation.” In contrast, the authors suggest an integrative mode that acknowledges a paradox and a developmental dialectic at play between solitude and relatedness, the product of which is a state of health requiring experiences of both. Stated simply, a positive experience of solitude is only possible when one is securely attached and secure attachment requires the potential for aloneness.