Hematopoietic stem cell transplantation (HSCT) remains an effective method for treating a multitude of malignant or nonmalignant disorders. While in autologous HSCT, patients receive their own stem cells after myeloablation, allogeneic HSCT implies using stem cells derived from a donor. It is often a last curative option, however, associated with a considerable risk of early and long-term complications. Acute complications determine the future course of illness, have a bearing on chronic complications, and overall quality of life. Similar to acute graft-versus-host disease, complications till day+100 posttransplant are included under acute complications. These include endothelial pathologies like engraftment syndrome (ES), transplant-associated thrombotic microangiopathy (TA-TMA), veno-occlusive disease of liver (VOD), capillary leak syndrome, and others which include noninfectious pulmonary complications, posterior reversible encephalopathy syndrome (PRES), infections (bacterial/fungal/viral), acute graft-versus-host disease (aGVHD), mucositis, and graft failure. It is important to recognize these complications at the earliest, for implementing suitable interventions, salvaging the graft and to prevent any disease relapse.