At present, there is minimal and misleading knowledge of gastrointestinal dysfunction in Parkinson's disease among primary neurologists and gastroenterologists. This book reviews our current understanding of the GI involvement in PD, as many experiments suggest that the gut might be the site where the disease starts. Also discussed is how the gut dysfunction causes symptoms from the top (mouth) to the bottom (anal canal), and because of their frequency and severity, they contribute to disease progression and disability. In addition, the book covers the negative effects of small intestinal bacterial overgrowth (SIBO) and of a frequently silent stomach infection by Helicobacter pylori infection on the absorption of antiparkinsonian drugs, in turn causing motor fluctuations and inadequate treatment response.   The Gut in Parkinson's Disease  serves as a bridge between patients, their movement disorders specialists, and other health care providers who currently provide compartmentalized and at times conflicting care for such needy patients. The book distills the clinical and research experience of the author, who -as a Clinical Professor of Medicine (Gastroenterology) at Stanford University School of Medicine over the past 20 years- managed hundreds of patients with Parkinson's Disease suffering from gut manifestations.