Thought insertion is the delusion that one's thoughts are not one's own, which causes people to believe that external agents have inserted ideas or thoughts into their minds. More prevalent in schizophrenia, thought insertion has been regarded as one of the most complex psychiatric symptoms. It is easy to see why it is such an intriguing phenomenon, as it blurs our understanding of some of the most fundamental aspects of our mind. Typically, discussions around thought insertion have tended to be featured in the context of philosophical examinations of broader issues in philosophy and psychiatry, or treated as a footnote to discussions of more prominent topics such as motor agency or the structure of phenomenal consciousness. For this reason, discussion of the phenomenon is incomprehensive and scattered throughout the literature, making it difficult to keep track of. Intruders in the Mind is an interdisciplinary attempt to bring together high-quality contributions to some of the most fundamental debates arising from the comprehensive study of thought insertion. Making thought insertion its central topic, this compilation gathers a series of essays that, taken as a whole, offer a broad and thoughtful approach to the clinical, phenomenological, conceptual, and experimental aspects of the systematic study of the phenomenon.