Application of numerical models at various scales and levels of sophistication increasingly enhances the value of the observations and improves the process understanding which is the fundamental basis for adequate monitoring. 2-dimensional or GIS-based spatial energy/mass balance models are used to assess the sensitivity of glacier mass balance. Such local model runs can be coupled with AOGCMs (Atmosphere-Ocean General Circulation Models) via RCMs (Regional Climate Models) in order to explore and explain large-scale and long-term variability. Such studies also provide essential information with respect to regional and global impacts (water resources, sea level). Continuity considerations for assumed step changes between steady-state conditions reached after the dynamic response time enable the reconstruction of decadal to centennial mass balances from cumulative length changes worldwide or back into historical and even Holocene times. In well-studied cases, dynamic fitting of time-dependent and coupled mass-balance/flow models to present-day geometries and observed long-term length change forms the basis for more detailed mass balance reconstructions and for extrapolations into a probably warmer world of the coming decades. Numerical modeling has, indeed, become an invaluable key element in modern glacier monitoring. For more details on the monitoring strategy, see literature.