Allogeneic Haematopoietic stem cell transplantation (HSCT) is an effective treatment for all array of blood or blood-producing organ disorders. Graft-versus-host-disease (GVHD) occurs as a result of an overactive immunological system against normal host tissues. It can happen in the liver, skin, mucosal surface of the eye, gastrointestinal tract, and genitalia. Ocular GVHD occurs in 30-70% of patients after HSCT. It mainly affects the ocular surface, including the conjunctiva and cornea. In severe cases, multiple clinical manifestations can lead to painful non-healing corneal ulcers, secondary infections, and visual loss. oGVHD can be debilitating and severely impact patients' quality of life. However, there are no widely accepted guidelines available for prevention and management. In collaboration with the Department of Haematology of Queen Mary Hospital, the investigators set out to establish a territory-wide cohort of patients receiving HSCT. Primarily, the investigators aim to establish the population-based epidemiology of oGVHD and understand the natural history and the long-term ophthalmic outcomes of oGVHD via this study.