This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides a summary of Pierre Janet’s central concepts related to hysteria and neuroses, as presented in his major works on the subject. It highlights Janet’s conception of subconscious linked to his theory on dissociation. The book also presents a Janetian interpretation of the pathological response to a psychological trauma as a passive effect of vehement emotions on higher mental functions. It shows that Janet’s phase-oriented treatment for patients with trauma-related disorders preceded modern approaches, and has much value for clinical practice. The book describes the importance of distinguishing desagregation from dissociation. It summarizes Janet’s clinical studies on the therapeutic relationship with traumatized patients, highlighting his approaches to somnambulistic influence and somnambulistic passion. The book also shows that Janet emphasized the importance of completing well-performed actions with a sense of joy.