This chapter describes a rational approach to septic arthritis based on sound principles of pathology and therapeutics. Early diagnosis and prompt treatment are essential if normal function of the infected joint is to be restored. In any acute joint disease, infection must be suspected, and if present, detected. It is important to identify causative bacteria in order to administer the correct antibiotic. The clinical features are different in neonates, children and adults. Diagnosis is not usually difficult and may be aided by diagnostic ultrasound and arthrocentesis. Both antibiotics and joint lavage are essential parts of management. Successful outcome is based on effective treatment and results in normal joint function. Ineffective or delayed treatment may be crippling.