Successfully treating shoulder arthroplasty infection requires diagnosis and bacterial identification. Higher incidence of infection with low-virulence bacteria makes this challenging. This study evaluates shoulder prostheses for infection using sonication and a functional biofilm assay. Nineteen patients undergoing revision shoulder arthroplasty were followed prospectively. Periprosthetic tissue and prosthetic components were obtained during the revision and evaluated with a functional biofilm assay. Results were compared with conventional cultures and laboratory results. Hardware samples were analyzed with scanning electron microscopy. Six of the 19 cases demonstrated growth on the biofilm assay. Three of these had positive conventional culture results and met Musculoskeletal Infection Society (MSIS) criteria for infection. Two other cases met MSIS criteria but demonstrated negative assay and conventional culture results. Of the six cases with positive assay results, three demonstrated evidence of biofilm on scanning electron microscopy. The biofilm assay identifies infections not recognized by traditional culture or MSIS criteria. (Journal of Surgical Orthopaedic Advances 27(3):171-177, 2018).