Publisher Summary This chapter discusses the dental plaque. The dental plaque is complex and contains a wide variety of micro-organisms, including those with little or no specific affinity for the true host oral surfaces but colonizing on the pre-existing flora. When exogenous micro-organisms enter the oral cavity, they first make contact with saliva and then colonize the various oral surfaces. If one of these surfaces cannot be colonized, the particular micro-organism will be eliminated by swallowing. Adherence is, therefore, the first necessary prerequisite for microbial colonization. Subsequent microbial survival then depends on their adaptive capacity to both the particular econiche and to changes in such a niche. Microbial succession is the term used to describe new microbial species superseding others, reflecting adaptive failure of the original flora. Subsequent oral microbial retention then depends on four factors: adaptation, growth, reproduction, and accumulation. The pathogenicity of dental plaque is dependent upon both the quality and quantity of its component microbial flora. Early plaque comprises mainly Gram-positive cocci, but this gradually gives way to a more filamentous Gram-positive flora. Subsequently, plaque matures to contain a complex microflora comprising a high proportion of Gram-negative micro-organisms.