Peer relationships among youth have been examined as predictors of mental health outcomes for at least fifty years, revealing dozens of discrete peer constructs that each are associated with adjustment in childhood, adolescence, and later in adulthood. Future research may benefit by examining a range of new outcomes and psychological processes that have been discussed recently in related literatures. This paper reviews recent research on interpersonal determinants of physical health outcomes, and opportunities for greater examination of 1) peer influence processes toward health risk behaviors; 2) neural correlates of peer adversity; 3) adverse peer experiences that may affect physiological markers of stress response; and 4) immune system markers of peer adversity. Additional future directions include the study of differences in the forms and functions of peer interactions within the digital age.