What is fatigue, and how can it be described and explained? Despite the fact that nurses often care for patients with fatigue, fatigue has not held a prominent place in nursing research and textbooks. This article presents an empirically developed theoretical model for understanding fatigue. Energy is used as a key term to describe and explain patients’ fatigue and related phenomena. Energy-generating and -economizing are suggested as central strategies for helping patients with fatigue. Nurses need to assess fatigue systematically in order to plan and carry out effective nursing interventions based on the patients’ individual symptom experience.