There are many facets to how and why catchment models are scale-dependent. Some examples will be presented to illustrate relationships between model structure and scale and what has been done to understand and to control model scale dependency. Also, some diagnostic tools for understanding model scale dependency using wavelet transforms and sensitivity analyses will be presented. Scale interactions between model and data errors will be illustrated using numerical experiments with a distributed model and some results from a recent Distributed Model Intercomparison Project (DMIP) experiment. Finally, scale interactions including model parameter estimation issues will be addressed, including results from recent Model Parameter Estimation Experiment (MOPEX) workshops.