Ectopic deciduosis represents an interesting phenomenon in both clinical medicine and pathology. It has been described as a physiological phenomenon of pregnancy and referred to as presence of decidual tissue outside of the endometrium or deposits of decidual cells outside of uterus. Although it is usually asymptomatic and incidental finding, ectopic deciduosis can result in serious complications and be life threatening if unrecognized and untreated adequately. Also, deciduosis can present with great variety of symptoms, lead to diagnostic difficulties and result in misdiagnosis. In this book we would like to introduce medical practitioners with process of decidual tissue formation, its purpose, ectopic decidual tissue localizations, its symptoms, diagnostic procedures, treatment and possible differential diagnoses and how to avoid confusing this condition with other ones.