In reality, most microorganisms are not free floating. They exist in biofilms, a community of many of them from the same species or from other genera and attached to surfaces.Microorganisms undergo a transition from free-floating, planktonic microorganisms to a sessile, surface-attached one. Contact with a surface induces changes in gene expression, and a strong attachment of microcolonies occurs only after a few hours. The maturation of a biofilm is associated with matrix formation. The matrix is of importance as it provides stability and protects against environmental insults, it consists of polysaccharides, water, lipids, proteins, and extracellular DNA. Biofilms can be found everywhere - in the environment, in water systems - and they play an important role in medicine and dentistry. In medicine, infections of chronic wounds, of the respiratory tract in cystic fibrosis infections, or when linked with incorporated biomaterial are mostly biofilm associated. In the oral cavity, the most prevalent oral diseases, dental caries, and periodontitis are multi-species biofilm-associated diseases. Although not acting alone, key pathogens drive the development of the microbial shift. Microorganisms metabolize sugar and create an acidic environment where aciduric bacteria (including mutans streptococci) become dominant, which leads to the demineralization of enamel and dentine. Porphyromonas gingivaliscauses biofilm dysbiosis in the development of periodontal disease. Biofilm-associated infections are extremely difficult to treat. The matrix serves as a barrier to antimicrobial agents and there are subpopulations of dormant bacteria resistant to antimicrobials requiring metabolically active cells. Approaches to treat biofilm-associated infections include the modification of the biofilm composition, inhibitors of quorum-sensing molecules, or interfering with matrix constituents.