The concept of historical range of variability in ecosystem structure or process is valuable in understanding and illustrating the dynamic nature of ecosystems; the processes that sustain and change ecosystems, especially disturbances; the current state of the system in relationship to the past; and the possible ranges of conditions that are feasible to maintain. Because ecosystems are structured hierarchically, historical range of variability must be characterized at multiple spatial scales and relevant time scales. Historical range of variability is a useful reference for determining a range of desired future conditions, and for establishing the limits of acceptable change for ecosystem components and processes.