Host modulation therapy (HMT) is a treatment concept that aims to reduce the tissue destruction and stabilize or even regenerate the periodontium by modifying or down regulating destructive aspects of the host response and up regulating protective or regenerative responses. The purpose of HMT is to restore the balance between proinflammatory and anti-inflammatory mediators, thereby protect mediators. This chapter covers the regulation of immune and inflammatory responses, excessive production of matrix metalloproteinases (MMPs), production of arachidonic acid metabolites, classification of host modulatory agents, and regulation of bone metabolism.