Periprosthetic joint infection is a rare, devastating complication. Common symptoms include persistent wound drainage, joint effusion, fever, acute/chronic joint pain, persistent stiffness, or decreased range of motion. Prompt diagnosis is critical for effective treatment. The preoperative workup includes a serum erythrocyte sedimentation rate and C-reactive protein, serum D-dimer, synovial fluid culture, synovial fluid cell counts with differential, synovial fluid leukocyte esterase, alpha-defensin testing, and synovial fluid erythrocyte sedimentation rate. A new diagnostic algorithm using has been developed to aid diagnosis. Once diagnosed, treatment may include debridement, antibiotics, irrigation and retention of implants; single-stage or 2-stage revision, and targeted antibiotics. © 2020 Elsevier Inc.