An overwhelming majority of known species of microorganisms form biofilms, i.e. spatially and metabolically structured communities embedded in the extracellular biopolymer matrix. Biofilm development is a complex multi-stage process involving reversible and, at a later stage, irreversible attachment of microbial cells to the substrate surface. Development also involves matrix formation, three-dimensional structuring of the whole community including the formation of mushroom- or pillar-shaped structures and, finally, the degradation of the biofilm and the dispersion of the cells involved. This book examines these processes in the example of microorganisms that interact with the animal or human organisms, playing the roles of symbionts or pathogens.