Trends in solid organ xenograft pathology are presented, with the focus on pig-to-nonhuman primate models. A simplified classification of rejection is followed, including hyperacute rejection (HAR), acute humoral xenograft rejection (AHXR), and acute cellular xenograft rejection (ACXR). The main components in HAR are natural xenoreactive antibodies in combination with complement activation. This is evident from the prevention of HAR in recipients in whom either antibodies or complement activation is depleted or inhibited. However, these strategies generally fail to prevent AHXR, which occurs later. AHXR is a multifactorial process in which natural and elicited antibodies may play roles, possibly in conjunction with complement, coagulation factors, and white blood cells. A main target appears to be the microvasculature which, in kidney grafts, is associated with a glomerular thrombotic microangiopathy. It is not clear to what extent species-specific physiologic disparities in complement and coagulation processes may play a role, separate from antibody-initiated processes. As rejection of solid organ xenografts is currently from AHXR, ACXR has not yet received close attention. In addition to intragraft rejection events, systemic complications following host-graft interactions have emerged, including (often fatal) consumptive coagulopathy and immune complex disease. It is anticipated that rejection processes will change when pigs with new genetic modifications become available. For instance, the precise role of natural antibodies to Galalpha1,3Gal will be able to be distinguished from other factors when pigs that lack the target antigen are available, and their organs can be evaluated in large animal xenotransplantation models.