This book provides a brief overview of the literature on the determinants of social isolation and its positive and negative effects. I examine the relationship between loneliness and being a loner. The relationship between loneliness and mental disorders such as depression, anxiety, post-traumatic stress disorder, schizophrenia, bipolar disorder, and personality disorders are also investigated. Furthermore, self-reports of various groups of loners are analyzed and discussed regarding their positive and negative experiences and the consequences these experiences had on their further development. It appears that many young people who were loners in their youth (many of whom were bullied and rejected) ended up later well in social and economic terms. However, many of them still appear to suffer to a greater or lesser extent from the negative experiences of social exclusion, bullying, and rejection. It is completely different for the category loners that voluntarily chose social isolation, often periodically, because they reported mainly about positive experiences. This book focuses specifically on information from case reports and an extensive self-report of a forensic psychiatric patient with mixed personality disorder in which the fatal effect of social exclusion becomes clear.