Fatigue is one of the most debilitating and common symptoms associated with hematologic and oncologic disorders and treatment strategies. Patient factors influence etiology and symptom manifestation, which in turn affect diagnosis, management, and monitoring. Fatigue instruments can assist with the identification, management, and monitoring of this difficult and debilitating process. A variety of fatigue instruments have been developed, several of which have been validated in patients with cancer. Psychometric confirmation is difficult because of the diverse nature of cancer-related fatigue. Prospective, comparative, and combination analyses of the developed instruments are required to confirm validation and clarify use in practice.