The aim of this chapter is to present the pathology of cardiac allograft rejection along with a brief historical background and summary of the clinical context and recent controversies. The intended audience is primarily pathologists, though it is also intended as a resource for heart failure/transplant investigators, clinicians, and surgeons seeking to better understand allograft biopsy interpretation. Different patterns of rejection are illustrated with emphasis on the most recent ISHLT working formulation classification schemes for cellular and antibody mediated rejection. An extensive discussion of immunopathology is also included. Nonrejection pathology encountered in transplant endomyocardial biopsies is addressed, including cardiac allograft vasculopathy. A brief section on the use of molecular pathology in rejection surveillance concludes the chapter.