Increased scarcity of old forests of natural origin, and improved understanding of the ways in which they are ecologically distinct, have led to a need for standardized definitions for old-growth forests in British Columbia, Canada. Useful definitions of old growth that are simple and concise have: been elusive because the old-growth stage of forest development occurs as a part of a continuum of development and varies greatly over ecological gradients at several scales. Therefore, we propose the development of ecologically based definitions in three broad categories: conceptual, quantitative., and working. We suggest that old growth be defined conceptually as a forest in which processes of gap dynamics predominate. We further propose that definitions of old growth should incorporate the distinct structural and compositional characteristics that arise from these developmental processes, because direct measures of gap dynamics are elusive and because structural attributes perform important ecological roles. Indices of forest structure that capture the inherent variability of structural characteristics of old-growth stands show promise for quantifying the qualitative features contained in conceptual definitions; minimum thresholds and demographic measures also have merit. In the interim, working definitions based on minimum age and height for different forest types have allowed the completion of province-wide inventories from existing databases. Future research should be directed toward determining the structural characteristics and variability of the different late-successional forest types in British Columbia so that conceptual definitions can be tested and type-specific ecological definitions can be developed. The paucity of basic information on characteristics of old-growth forests limits the ability of managers to determine the area and location of specific types of old-growth forest, and to develop appropriate management goals and methods.