An enormous number of bacterial species exist in nature and the human environment with important roles in natural chemical and biological cycles involved in the agricultural aspects of food and industrial activity. However, only a relatively limited number of microbes are recognized as important pathogens for humans and causes of clinical infections, including well-known species like Salmonella spp.,Streptococcus pyogenes, or Corynebacterium diphtheriae. Development of newer microbiologic techniques permitted significant changes in medicine over the last half century, including development of newer antibacterial agents, advanced surgical procedures and development of intensive care units (ICUs) in hospitals. New organisms have attained an increasingly greater attention of clinical microbiologists, biomedical researchers and clinicians, especially in the ICUs. These microbes are Gram-negative bacteria and have an important role in nosocomial infections. During the last few decades, Acinetobacter spp. have been implicated in a wide spectrum of infections, e.g., bacteremia, nosocomial pneumonia, urinary tract infections, secondary meningitis, and superinfections in burn patients. One of the most striking features of Acinetobacter spp. is their extraordinary ability to develop multiple resistance mechanisms against major antibiotic classes. They have become highly resistant to broad spectrum s-lactams (third-generation cephalosporins, carboxypenicillins, and to carbapenems). They produce a wide range of aminoglycoside-inactivating enzymes and most strains are resistant to fluoroquinolones. Acinetobacter are now known to be important causes of nosocomial infections, a major problem confronting ICU clinicians and this is related to the severity of infections and development of multiple drug resistance by these organisms to major antibiotic classes. Therefore, it seems important to review the rapidly expanding knowledge and major characteristics about these important organisms and update previously published books (“The Biology of Acinetobacter”, edited by K.J. Towner, E. Bergogne-Berezin and C.A. Fewson, 1991 Plenum Press” and “Acinetobacter, Microbiology, Epidemiology, Infections, Management, edited by E. Bergogne-Berezin, M.L. Joly-Guillou, K.J. Towner, 1996 CRC Press).