The gingival tissues are exposed to a wide variety of adverse enviromental factors in the oral cavity. During mastication of food the bolus passes over the gingival surface with force. The gingiva is also exposed to extreme variations in temperature and pH. Over 300 bacterial species, some of them pathogenic, have been recognized as commensals of the oral cavity. In spite of all these adverse environmental conditions, periodontal tissues remain in a state of health, normally. This is because of the host defence system which strikes a balance with these adversities. Host defence is established at various levels. In this book various defence mechanisms of gingiva are discussed.