Dissociative identity disorder (DID) is life-long, childhood-onset, posttraumatic developmental disorder where chronic early-life maltreatment and attachment disturbances prevents the child’s development of a continuous sense of self across emotional states, relationships, and social contexts. As development proceeds, these self-states acquire a sense of themselves, a capacity for information processing, memory, emotion, and behavior. Conceptualizing DID involves paradoxes and apparent contradictions. DID has been categorized as a severe mental illness with major psychiatric comorbidities. Studies show that DID individuals have a unique personality organization with repeated, often covert posttraumatic reactivity, especially in relationships (e.g., therapy). Paradoxically, research shows that, during development, DID individuals preserve psychological resiliencies consistent with responsivity to long-term, psychodynamically informed treatment. These include, when not stressed, capacities for therapeutic alliance, reality testing, and observing ego.