Reasoning is a mental operation in which, with predefined judgments, we generate a new judgment. The artificial reasoning systems are any software application, hardware device or combination of software and hardware whose computational function is to generate conclusions from available knowledge using logical techniques of deduction, induction or other forms of reasoning. This book is focused on presenting some theory of artificial reasoning systems and some applications in medicine. The book contains two parts the first one is a deep description of many theoretical aspects in the domain of artificial reasoning as well as Case Based Reasoning, Distributed Reasoning through agents and Reasoning under Uncertainty with Fuzzy Systems. In the second part we describe some medical applications for Decision Support and Diagnosis, Explanation Aware Computing and Document Recommendation System for Health 2.0.