Infectious processes about the knee can result from the hematogenous spread of infection, spread from a contiguous source, direct implantation of pathogens, and prior surgery. Soft tissues, joints, and bones can be infected by bacteria, fungi, parasites, or viruses. MR imaging is useful in identifying cellulitis, abscess, septic arthritis, and osteomyelitis. The inherent tissue contrast provided by MR imaging allows for the delineation of soft-tissue infection and osteomyelitis. Therefore, MR imaging is a useful tool in evaluating the extent of infection, and in facilitating adequate debridement and drainage. MR imaging is particularly useful in the setting of chronic posttraumatic osteomyelitis and in prior surgical procedures, such as arthroscopy, anterior cruciate ligament reconstruction, and amputation.