Periodontitis is a complex multifactorial disease that involves complex interactions between pathogenic microorganisms and host responses. In the past few decades, advancements in basic research have brought considerable changes in disease pathogenesis. Based on the same, various models of periodontal pathogenesis have been proposed. Right from the basic linear models of periodontal progression, which states that periodontal disease is the result of the microbial challenge and toxic products to the recent models like the biologic system model, the keystone pathogenesis model, etc which discusses the progression of periodontal disease in terms of intricate host-microbial interactions which is complicated by various risk factors, our perception about the progression and pathogenesis has tremendously changed. This book gives a comprehensive review of the models of periodontal pathogenesis and attempts to highlight the important aspects of evolving concepts in periodontal disease progression.