Subdural empyema represents loculated infection between the outermost layer of the meninges, the dura, and the arachnoid. The empyema may develop intracranially or in the spinal canal. Intracranial subdural empyema is most frequently a complication of sinusitis or, less frequently, otitis or neurosurgical procedures. Spinal subdural empyema is rare and may result from hematogenous infection or spread of infection from osteomyelitis. The most common organisms in intracranial subdural empyema are anaerobic and microaerophilic streptococci, in particular those of the Streptococcus milleri group (S. milleri and Streptococcus anginosus). Staphylococcus aureus is present in a minority of cases, and multiple additional organisms, including Gram-negative organisms, such as Escherichia coli, and anaerobic organisms, such as Bacteroides, may be present. Pseudomonas aeruginosa or Staphylococcus epidermidis may be present in cases related to neurosurgical procedures, and Salmonella species have been detected in patients with advanced AIDS; multiple organisms may be present simultaneously. Spinal subdural empyemas are almost invariably caused by streptococci or by S. aureus. Subdural empyema--whether it occurs in the skull or the spinal canal--may cause rapid compression of the brain or spinal cord, and represents an extreme medical and neurosurgical emergency. The diagnostic procedure of choice for intracranial and spinal subdural empyema is MRI with gadolinium enhancement. Computed tomography scan may miss intracranial subdural empyemas detectable by MRI. Conversely, occasion spinal subdural empyemas may be detected by CT myelography where MRI is negative. Treatment in virtually all cases of intracranial or spinal subdural empyema requires prompt surgical drainage and antibiotic therapy. Pus from the empyema should always be sent for anaerobic, as well as aerobic, culture. Because intracranial subdural empyemas may contain multiple organisms, provisional antibiotic therapy of intracranial subdural empyema, where the organism is unknown, should be directed against S. aureus, microaerophilic and anaerobic streptococci, and Gram-negative organisms. Antibiotics should include 1) nafcillin, oxacillin, or vancomycin; plus 2) a third generation cephalosporin; plus 3) metronidazole. Provisional antibiotic therapy of spinal subdural empyemas should be directed against S. aureus and streptococci, and should include nafcillin, oxacillin, or vancomycin. Morbidity and mortality in intracranial and spinal subdural empyema relate directly to the delay in institution of therapy. Both conditions should, thus, be treated with great urgency.