Dental plaque is an adherent intercellular matrix consisting primarily of proliferating microorganisms, along with a scattering of epithelial cells, leukocytes and macrophages. This chapter covers the dental plaque as a biofilm, nature of dental plaque, types of dental plaque, clinical assessment of dental plaque, composition of dental plaque, formation or development of dental plaque, structural and microscopic properties of plaque, clinical significance of plaque, microbial specificity of periodontal diseases, periodontal tissue damage, microorganisms associated with periodontal diseases, key characteristics of specific periodontal pathogens, and types of infection. Structurally, dental plaque is now considered to be a biofilm of complex and dynamic microbial community. Based on its relationship to the gingival margin, plaque is differentiated into two categories following as supragingival and subgingival plaque, which are briefly explained in this chapter.