The Secret - we think of the clandestine, the concealment and hiding, of the secret that children need for their development, which can also become unsettling when they have to keep it to themselves. However, the secret is also a mystery and something enigmatic - such as the mystery of nature, healing, or inspiration. In this sense, it represents the other and the foreign, that which eludes us and what we cannot fully understand. The authors of this book explore the secret in its broad range of meanings. The contributions span from clarifying the concept to the psychology and psychopathology of secrecy, leading to enigmatic phenomena from psychiatry, and extending to culture, visual arts, and literature, where the mysterious finds its diverse expression.