There is a great deal of apparently well, but in fact medically compromised patients whose illnesses might be undetected or inadequately treated. This group of patients is at the greatest risk of developing complications when undergoing oral surgery. Thus, application of some general principles is advised in order to avoid fatal complications, principles such as: knowledge of the medical history of all patients, knowledge of potential interactions between medical conditions and dental treatment and knowledge of medical emergency procedures. To provide care to patients with a cardiovascular disease, dentists must be aware of its treatment and approach with caution. Patients with coagulopathy, endocrine disorders, abnormalities of the immune system and disorders of the renal system are regulated primarily by laboratory results which indicate the type of dental treatment needed. Antibiotic prophylaxis is justified only in certain situations for post-splenectomy patients and patients with total hip arthroplasty and if there is a risk of developing infective endocarditis. Osteoporosis, chemotherapy and radiotherapy imply the presence of agents that can lead to oral complications such as mucositis, infections, saliva changes etc. Oral surgery in such conditions can lead to necrosis of the jaw. Therefore, oral assessment and dental care are highly recommended before such therapies to eliminate all potential sources of oral infections and trauma. For a pregnant patient certain precautions in the use of drugs and radiographs must be taken. Allergies are a result of an excessive response of the immune system to an allergen. In dental practice, they are associated with certain drugs and materials.