The aim of this review article is to examine the associations between periodontal diseases and common systemic diseases, namely diabetes, respiratory diseases, cardiovascular diseases and osteoporosis. A substantial number of review articles have been published to elucidate the relationships between these diseases; however, none provide a complete overview on this topic from the aspects of definition, classification, clinical characteristics and manifestations, inter-relationships and interactions, proposed schematic mechanisms, clinical implications and management of periodontal patients with these systemic diseases. The aim of this article is to provide an overall understanding and general concepts of these issues in a concise and inter-related manner.