Production of ?-lactamase enzymes that break down the structural ?-lactam ring of penicillin-like drugs considered as a predominant mechanism of bacterial resistance to ?-lactam antibiotics. Extended- spectrum ?- Lactamases (ESBLs) have been found in many pathogenic Gram-negative bacteria, but they are most common in nosocomial isolates of K. pneumoniae. Bacteriocins are antibacterial proteins produced by bacteria. The bacteriocins (klebocins) of K. pneumoniae are closely related to the colicin family. Although K. pneumoniae is one of the well known bacterial species in which the ESBLs and bacteriocin have been most commonly reported around the world, studies in Iraq by many researchers were focused on epidemiology, pathogenicity and serology of Klebsiella spp. but, little or no attention has been paid to ESBL and bacteriocin production in Klebsiella spp. This book provides a new data on prevalence of ESBL and bacteriocin- producing environmental and clinical K. pneumoniae isolates in Iraq, the heterologous activity of klebocin produced, and the genetic factors controlling ESBL and bacteriocin production in these isolates.