Theory of Mind is the ability to impute mental states (intentions, beliefs, thoughts, desires, emotions and other psychological states) to the self e to the others as a way of making sense and predicting behavior. This work is aimed to analyze deeper the development of mental states understanding in children and its link with emotional difficulties. It deals with some novel topics, regarding Theory of Mind assessment in school age children and the relationship between Theory of Mind and internalizing problems. The first chapter reviews the main theoretical approaches and the relationship between Theory of Mind and social/emotional functioning. The second chapter presents the development of Theory of Mind and discusses the methodological issue, concerning the assessment of epistemical and emotional states understanding in school children. Finally, it is studied the relationship between Theory of Mind and psychological risk in children, pointing out the link between poor Theory of Mind and frequent somatic complaints in a normal school age children population.