Fatigue in adults with cancer has received considerable attention as a troublesome symptom that requires nursing intervention. Fatigue in children with cancer, however, has received considerably less focus. The first phase of the present study used qualitative methods to generate a detailed description of fatigue in children with cancer. Thirteen children (ages 5 to 15) and 12 parents from the oncology service in two regional children’s hospitals participated in the initial interviews; a validation sample comprised another 7 children and 6 parents from a third site. Transcribed interviews were subjected to grounded theory analysis. Energy, as an overriding phenomenon, was a core concept in the descriptions of fatigue. Findings suggest that children with cancer may experience three subjectively distinct types of fatigue that represent different levels of energy: typical tiredness, treatment fatigue, and shutdown fatigue. Children managed their dwindling energy and minimized further energy loss through strategies of replenishing, conserving, and preserving. Children’s use of these strategies was influenced by temperament, lifestyle, environmental factors, and treatment modalities. Knowledge of the specific types of fatigue in children can offer direction for optimal intervention and for further research.