Transplant rejection is a fundamental concept within immunology and transplantation science, defining the complex biological processes by which the recipient's immune system identifies and attacks a transplanted organ or tissue as foreign, leading to graft dysfunction or failure. This field of study investigates the intricate cellular and molecular mechanisms underlying the immune response to alloantigens, characterizes different types of rejection based on timing and immunological pathways, and explores strategies for immunosuppression, tolerance induction, and diagnostic monitoring to prevent or mitigate this critical barrier to successful transplantation and patient survival.