Many systemic (infective, genetic, autoimmune, neoplastic) diseases may involve the oral cavity and, more generally, the soft and hard tissues of the head and neck as primary or secondary localization. Primary onset in the oral cavity of both pediatric and adult diseases usually represents a true challenge for clinicians; their precocious detection is often difficult and requires a wide knowledge but surely results in the early diagnosis and therapy onset with an overall better prognosis and clinical outcomes. In the current paper, as for the topic of the current Special Issue, the authors present an overview on the most frequent clinical manifestations at the oral and maxillo-facial district of systemic disease.