Many cephalosporins are now in clinical use. They have a wide range of activity against different species of bacteria and also differ in their pharmacokinetic and metabolic characteristics. This article outlines a basis for comparison and classification of cephalosporins into 4 groups active mainly against: (1) Gram-positive bacteria; (2) Gram-negative rods; (3) Gram-negative rods including Pseudomonas spp.; and (4) Gram-negative rods including Bacteroides spp. Further subdivisions highlight the pharmacological differences.