Biofilms are complex, differentiated communities of prokaryotic cells that form in diverse ecosystems. These communities consist of highly structured, multispecies communities where metabolic activities are integrated, and developmental sequences similar to those of multicellular organisms can be observed. Biofilms differ significantly from planktonic cells in their gene expression and phenotypic characteristics. Molecular and microscopic evidence suggests the existence of a sequence of biofilm phenotypes. Complex cell-cell interactions in prokaryotic communities are an ancient characteristic, facilitated by the localization of cells at surfaces. Surfaces may have provided the protective niche for attached cells to create a localized homeostatic environment. Biofilm and planktonic phenotypes may be viewed as integrated components of prokaryote life. Biofilm formation involves initial attachment, irreversible attachment, maturation, and detachment. Biofilms develop complex structures with microcolonies separated by water channels. The structure of biofilms is influenced by hydrodynamics, with higher shear leading to elongated cell clusters. Biofilm structure is largely determined by the production of extracellular polymeric substances (EPS), which provide structural support. EPS is composed of polysaccharides, proteins, and nucleic acids. The production of EPS is influenced by environmental factors and is essential for biofilm development and function. Cell-cell communication during biofilm formation is crucial for the development of complex structures. Quorum sensing, mediated by acyl homoserine lactones (AHLs), plays a key role in regulating gene expression and biofilm development. The RpoS sigma factor is involved in stress response and biofilm development. Detachment is a component of biofilm development, allowing cells to return to a planktonic mode of growth. Biofilm formation is a developmental process involving multiple stages, with distinct physiological states for different stages of biofilm development. Biofilms are physiologically integrated microbial communities, with cells working together to form efficient, cooperative systems. Biofilms are behaviorally integrated communities, with cells communicating and coordinating their activities. Biofilms are highly structured communities, with cells arranged in specific patterns that facilitate physiological cooperation. Biofilms are self-assembling communities, with cells organizing themselves through coordinated developmental processes. The genomes of many bacterial species are expressed in various phenotypes, and the proteomic data show the profound changes in gene expression that occur during the transition from planktonic to biofilm growth. These changes allow bacteria to adapt to different environments and form complex, integrated communities.Biofilms are complex, differentiated communities of prokaryotic cells that form in diverse ecosystems. These communities consist of highly structured, multispecies communities where metabolic activities are integrated, and developmental sequences similar to those of multicellular organisms can be observed. Biofilms differ significantly from planktonic cells in their gene expression and phenotypic characteristics. Molecular and microscopic evidence suggests the existence of a sequence of biofilm phenotypes. Complex cell-cell interactions in prokaryotic communities are an ancient characteristic, facilitated by the localization of cells at surfaces. Surfaces may have provided the protective niche for attached cells to create a localized homeostatic environment. Biofilm and planktonic phenotypes may be viewed as integrated components of prokaryote life. Biofilm formation involves initial attachment, irreversible attachment, maturation, and detachment. Biofilms develop complex structures with microcolonies separated by water channels. The structure of biofilms is influenced by hydrodynamics, with higher shear leading to elongated cell clusters. Biofilm structure is largely determined by the production of extracellular polymeric substances (EPS), which provide structural support. EPS is composed of polysaccharides, proteins, and nucleic acids. The production of EPS is influenced by environmental factors and is essential for biofilm development and function. Cell-cell communication during biofilm formation is crucial for the development of complex structures. Quorum sensing, mediated by acyl homoserine lactones (AHLs), plays a key role in regulating gene expression and biofilm development. The RpoS sigma factor is involved in stress response and biofilm development. Detachment is a component of biofilm development, allowing cells to return to a planktonic mode of growth. Biofilm formation is a developmental process involving multiple stages, with distinct physiological states for different stages of biofilm development. Biofilms are physiologically integrated microbial communities, with cells working together to form efficient, cooperative systems. Biofilms are behaviorally integrated communities, with cells communicating and coordinating their activities. Biofilms are highly structured communities, with cells arranged in specific patterns that facilitate physiological cooperation. Biofilms are self-assembling communities, with cells organizing themselves through coordinated developmental processes. The genomes of many bacterial species are expressed in various phenotypes, and the proteomic data show the profound changes in gene expression that occur during the transition from planktonic to biofilm growth. These changes allow bacteria to adapt to different environments and form complex, integrated communities.