Periodontitis is the inflammatory disease caused by periodontal pathogens in dental plaque, and progression of this pathological condition is also considered as a major cause of alveolar bone resorption and subsequent tooth depletion. LPS and several kinds of proteinases produced by periodontal pathogens directly destroy periodontal tissues. On the other hand, host defense systems existing in the periodontal tissues exert essential roles in protection of periodontal tissues from bacterial invasion. Immune system divides broadly into 2 categories, innate immune and acquired immune systems. The former is the first defensive barrier against infectious diseases of pathogens, and phagocytic cells including macrophages and neutrophils are involved in this immune system as innate immune function. The latter is the secondary immune system that antigen-presenting cells such as dendritic cells detect smaller pathogens or intracellular pathogens, and antigens which retain as intact proteins on the surfaces of these cells activate T cells and B cells. However, once inflammation becomes persistent with bacteria, these biological defective mechanisms causes breakdown of host innate and acquired immune systems, and subsequent destruction of periodontal tissues. Here we review the mechanisms by which periodontal pathogens cause bone resorption.