Significant changes have taken place in the epidemiology, microbiology and antibiotic therapy of bone and joint infections. Gram-negative bacilli have become an increasingly common cause, particularly in immunocompromised patients; anaerobes have been implicated in osteomyelitis associated with metallic foreign bodies; and there is increasing use of oral antibiotic regimens following an initial period of parenteral treatment. Gram-negative bacilli and anaerobes are found in polymicrobial non-haematogenous osteomyelitis (e.g. post-traumatic, post-surgical), but Staphylococcus aureus remains the most common cause of acute haematogenous osteomyelitis, with streptococci and Haemophilus influenzae responsible for most of the remainder. A precise microbiological diagnosis is essential. Diagnosis is based on Gram stain and culture of bone biopsies or aspirated pus, or on blood cultures. Specimens should be obtained before starting therapy. Any suspected primary foci of infection should be cultured. Parenteral antibiotics are given as soon as specimens are obtained, and continued for at least 3 weeks. The common causative organisms in septic arthritis are the same as in osteomyelitis, with the addition of Neisseria gonorrhoeae in young, sexually active adults. As in osteomyelitis, a precise microbiological diagnosis is of paramount importance, ideally by joint aspiration for cell count, Gram stain, biochemical analysis and culture, or by blood cultures. Optimum therapy is with antibiotics, repeated therapeutic aspirations, and resting the joint. Parenteral antibiotics should be started as soon as specimens are obtained and continued for 4 to 6 weeks. Gonococcal arthritis, however, can be treated successfully with 1 week of antibiotics. When treatment of either osteomyelitis or septic arthritis is continued with oral antibiotics, serum antibiotic concentrations or serum bactericidal levels are mandatory to ensure adequate absorption.