Clinical relevance: Mucositis is, without a doubt, one of the most common and debilitating complications resulting from the intense cytotoxic therapies preceding hematopoietic stem cell transplantation. Mucositis is not only a problem for the patient who needs to deal with the pain and the rapidly decreasing quality of life, but it is also a problem for the health system. Mucositis frequently results in increased hospital length of stay, resulting in a significant increase in the cost of medical care. Multiple prophylactic and therapeutic attempts have been made in order to control the symptoms and the severity of mucositis. No prevention or cure is available for the disease. Despite the renewed interest in this complication, important gaps in knowledge remain in this area of the supportive care of cancer patients. These gaps in knowledge are wider in the area of mucositis in patients who undergo hematopoietic stem cell transplantation.