The oral cavity is an important anatomical location with a role in many critical physiologic processes. It is unique for the presence of exposed hard tissue surrounded by mucosa. Truly the oral cavity is a mirror that reflects and unravels many of the human body's internal secrets. The oral cavity might well be thought as window to the body because oral manifestations accompany a wide array of systemic diseases. In many instances, oral involvement precedes the appearance of other symptoms or lesions at other locations. These oral manifestations must be properly recognized if the patient is to receive appropriate diagnosis and referral for further treatment. This article is intended to highlight the lesions of oral mucosa, dentition, salivary glands, facial skeleton, extra-oral skin and other related structures that related to or are caused by some of the more common systemic diseases, and hope to provide ample insight for physicians, dentists, and clinicians