The plant functional types (growth forms) traditionally recognized by arctic ecologists provide a useful framework for predicting vegetation responses to, and effects on, ecosystem processes. These functional types are similar to those objectively defined by cluster analysis based on traits expected to influence ecosystem processes. Principal components analysis showed that two major suites of traits (related to growth rate and woodiness) explain the grouping of species into functional types. These plant functional types are useful because they (1) influence many ecological processes (e.g. productivity, transpiration, and nutrient cycling) in similar ways, (2) predict both responses to and effects on environment, including disturbance regime, and (3) show no strong relationship with traits determining migratory ability (so that no functional type will be eliminated by climatic change simply because it cannot migrate). Circumstantial evidence for the ecological importance of these functional types comes from the distribution of types along environmental gradients and the known ecological effects of traits (e.g., effects of litter quality on decomposition and of plant height on winter albedo) that characterize each functional type. The paleorecord provides independent evidence that some of these functional types have responded predictably to past climatic changes. Field experiments also show that plant functional types respond predictably to changes in soil resources (water and nutrients) but less predictably to temperature. We suggest that evidence for the validity of arctic plant functional types is strong enough to warrant their use in regional models seeking to predict the transient response of arctic ecosystems to global change.