Holistic modeling embeds water resources and economic sub-models into a single consistent framework and can present a simple approach for building truly integrated water resources and economic models. In addition to the integration of hydrologic-economic approaches, holistic modeling can help address combined environmental-economic issues. At the same time, holistic modeling faces a series of challenges related to data availability, model formulation, and solution approaches. The model formulation typically needs to integrate different temporal and spatial scales inherent to the disciplines integrated into the modeling framework. Moreover, the size and complexity of a holistic model often exceeds the capabilities of existing solution algorithms. This paper will discuss these challenges. A prototype integrated hydrologic-agronomic- economic model for a basin water allocation study is presented as an example of how various spatial and temporal scales can be integrated, and to demonstrate possible solution approaches to large and complex holistic modeling. Copyright ASCE 2004.