Biofilms are a ubiquitous life style of microbes. Every surface covered with water will develop microbial biofilms. Biofilms are notorious for strong resistance to antimicrobial agents and for causing serious problems in modern medicine and industry. Biofilms are consortia composed of mushroom-shaped clumps of bacteria bound together by a polysaccharide matrix and have many water channels which deliver nutrients and remove wastes. In experiments using Pseudomonas aeruginosa, the genesis of such biofilm architecture was shown to be under the control of the quorum-sensing system. A mutant defective in the quorum-sensing system established a thin uniform biofilm that was easily disrupted by detergent. Thus, the quorum-sensing (a cell-to-cell communication) system working in a collective behavior of bacteria was shown to be important. With respect to the strong resistance of biofilms to antimicrobial agents, heterogeneous distribution and physiology of biofilm cells under various stresses were indicated as factors to be considered, in addition to the exopolysaccharide barrier to chemicals. Generation of drug resistant mutants from biofilms was suggested by recent arguments on the hypermutability of cells under stress.