The oral cavity is the beginning of the digestive system. It is the front door of the organism. Its structure is adapted to its function, and it also protects the whole organism through numerous defense factors. Oral health and general health are interlinked in many various manners. The vast majority of systemic diseases will also manifest in the oral cavity. The first clinically visible changes caused by most systemic diseases will appear in the oral cavity and sometimes these changes are also the only manifestation of the underlying disease. Signs of diabetes in the oral cavity are multiple and directly related to the degree of control of the disease. Complications of diabetes in the oral cavity include acetone breath, dry mouth, impaired sense of taste, sialadenosis, burning mouth syndrome, oral infections, candidiasis, dental caries, periodontal disease and alveolar bone changes. Gastroesophageal reflux disease is a pathological condition in which the stomach contents leak backwards from the stomach into the esophagus. Chronic exposure of the oral cavity to the acidic content causes erosion of dental hard tissues and also affects the soft tissues causing inflammation. Crohn's disease is a chronic inflammatory disease of the gastrointestinal system in which changes in the oral cavity may occur even several years prior to the inflammatory changes of the intestines. Clinical changes include occurrence of edema of the lips and face and a cobblestone appearance of the oral mucosa. Pemphigus is an autoimmune blistering disease that affects the skin and mucous membranes. The first sign of the disease in the oral cavity is the appearance of blisters that burst quickly leaving behind erosions and ulcerations. The predominant clinical sign of all types of anemia in the oral cavity is the paleness of the mucous membranes. Due to the loss of lingual papillae the tongue gets a smooth and bald appearance that is accompanied by the occurrence of angular cheilitis. In most cases, the first visible clinical changes caused by systemic diseases first affect the oral mucosa and they can be easily detected during routine dental examinations. Therefore, it is essential that doctors of dental medicine are introduced to oral manifestations of systemic diseases.