Breast cancer treatment is dependent on accurate pathologic diagnosis. HER2 testing is now universally recommended as part of evaluation of invasive breast cancer. HER2 testing is available via various slide and non-slide based assays, and interpretation of results continues to evolve. Herein we review these testing modalities and their incorporation into the 2013 ASCO/CAP guidelines. Once accurate HER2 status has been established the proper treatment based on recent clinical trials can be instituted.