Theory of Mind, the cognitive capacity to attribute emotions, intentions and knowledge to oneself and others, has been claimed a hallmark of human cognition. Nonetheless, Theory of Mind is considered limited in young children and people with autism. Moreover, its presence in animals is much investigated, and hotly debated. Essential for cross-disciplinary discussions and real insight in this unique capacity is knowing what constitutes Theory of Mind. We aim to tackle this by combining insights from three different scientific fields that study Theory of Mind: animal behaviour, typical child development and developmental disorders such as autism and AD/HD.