The long historical record and extensive modern observations for the Vernagtferner make the glacier a prime object for modelling of its present state and past fluctuations. Historically, the glacier appears to have passed through two radically different phases, characterized by cyclic surging and by shrinkage back into higher accumulation-favorable regions, respectively. A unified computer simulation of this bimodal behavior on a highly complex bedrock topography remains beyond the power of existing glacier models. However, the essentials of the two phases have been reproduced. It appears that the surging mode of flow could operate only as long as the glacier remained large enough to create substantial basal melting through the combination of high base stresses and rapid flow rates. The velocity and thickness profiles observed during the retreat phase are shown to be well simulated by a deformational flow model including parameterizations of the most essential three-dimensional features. The great retreat of the glacier since 1848 appears to have resulted from a climatic mass balance decrease over its entire surface of order 0.2 rna - I superimposed on an enlarged post-surge ablation region. This interpretation holds the potential for future readvance, and perhaps an eventual return to the surging mode, foreshadowed by the recent general thickening of the Vernagtferner.