The beta lactam antibiotics have a b-lactam ring. Penicillins, cephalosporins, monobactams and carbapenems are b-lactam antibiotics. They act by inhibiting the cell wall synthesis in the bacteria. Penicillin G has a narrow antibacterial spectrum and is the antibiotic of choice for several infections unless the patient is allergic to it. Semisynthetic penicillins are acid stable, some are penicillinase resistant and some of them have an extended antibacterial spectrum. Carboxypenicillins are effective against pseudomonas while ureidopenicillins are effective against pseudomonas as well as klebsiella infections. b-lactamase inhibitors prevent the destruction of b lactam antibiotics and extend their spectrum. Cephalosporins are b-lactam antibiotics derived from cephalosporin C and have a wider spectrum of activity than penicillins. They are classified into four generations and are one of the commonly used groups of antibiotics. Carbapenems contain a b-lactam ring fused with a five membered penem ring. They are used in respiratory, skin, bone, soft tissue, intra-abdominal, and gynecological infections. Monobactams are monocyclic beta-lactams and include aztreonam which is effective against gram negative bacilli including Pseudomonas aeruginosa.