While considerable progress has been made in the development of global flux products from space (see Chapter 10), there remain a number of issues that either limit the application of these data to their fullest extent or provide an inherent constraint on the accuracy achievable. This is particularly true when using remote sensing-based information but is also pertinent to those approaches driven by reanalysis or produced via other modelbased meteorological data streams or outputs, such as land surface, regional, or global climate model (GCM) simulations. These issues fall largely (but not exclusively) into two categories: (1) model physics and structural limitations (including the provision of information to drive the models) and (2) model assessment and interpretation of simulations.