The skin is the most visible and easily accessible organ of the body. Therefore, as a readily available indicator, it can give a clue to many underlying systemic diseases ; it has great diagnostic implications, serving as a window to the diseases affecting the internal organs. Recognition of dermatological manifestations of systemic diseases is practically important for all practicing clinicians. In many instances, the recognition of the skin signs of systemic diseases should minimize the cost of diagnostic testing. Generally, skin symptoms are quite commonly associated with systemic disease. Various skin manifestations have been observed in patients with either benign of malignant systemic diseases, and they often give a clue to or confirm the underlying diseases. This lecture summarizes the cutaneous manifestations and specific skin signs that help to differential diagnoses of systemic diseases.