Recent public announcements stated that 60% to 85% of all microbial infections involve biofilms developed on natural tissues (skin, mucosa, endothelial epithelia, teeth, bones) or artificial devices (central venous, peritoneal and urinary catheters, dental materials, cardiac valves, intrauterine contraceptive devices, contact lenses, different types of implants). Prosthetic medical devices are risk factors of chronic infections in developed countries and these infections are characterized by slow onset, middle intensity symptoms, chronic evolution and resistance to antibiotic treatment. In case of biofilm development, a series of genes (40-60% of the prokaryotic genome) are modulated (activated/inhibited) by complex cell to cell signalling mechanisms and the biofilm cells become phenotypically distinct from their counterpart--free cells, being more resistant to stress conditions (including all types of antimicrobial substances); this resistance is phenotypical, behavioural and, more recently, called TOLERANCE. Four major mechanisms can account for biofilm antibiotic tolerance: (1) the failure of antibiotic penetration into the depth of a mature biofilm due to the biofilm matrix; (2) the accumulation of high levels of antibiotic degrading enzymes; (3) in the depth of biofilm, cells are experiencing nutrient limitation entering in a slow-growing or starved state; slow-growing or non-growing cells being not highly susceptible to antimicrobial agents, this phenomenon could be amplified by the presence of phenotypic variants or "persisters" and (4) biofilm's bacteria can turn on stress-response genes and switch to more tolerant phenotypes on exposure to environmental stresses; (5) genetic changes, probably selected by different stress conditions, such as mutations and gene transfer could occur inside the biofilm. In these conditions, biofilm associated infections require a different approach, both clinically and paraclinically.