Children undergoing hematopoietic stem cell transplant often require intensive care support due to their underlying disease, sepsis, infection, hemorrhage, respiratory failure and organ dysfunction. The majority of children requiring intensive care support have an allogeneic donor. These children carry a higher likelihood of graft versus host disease complicating their medical management. Understanding the process of graft versus host disease is important in the shared care of these children between pediatric intensive care physicians and the bone marrow transplant team.