Allogeneic hematopoietic stem cell transplantation (HSCT) has become an established treatment modality for various hematological diseases. However, in allogeneic HSCT, patients often suffer from severe gastrointestinal complications caused by the conditioning regimen and acute/chronic graft-versus-host disease, which requires support by multidisciplinary nutritional support teams (NST). In addition, pretransplantation nutritional status can affect the clinical outcome after allogeneic HSCT. Therefore, it is important to refer the patient to a NST when becoming aware of nutritional problems before allogeneic HSCT. It is also important to follow nutritional status over the long term, as patients often suffer from various nutritional problems, such as malnutrition and metabolic syndrome, even late after allogeneic HSCT. In summary, NST can contribute to the improvement of nutritional status and possibly prognosis at every stage before and after allogeneic HSCT. Here, we aim to give a comprehensive overview of current understanding about nutritional support in allogeneic HSCT and try to provoke a constructive discussion to stimulate further investigation.