Sediment transport modeling is notoriously difficult. Sediment data are uncertain and transport theory is empirical and highly sensitive to a broad array of physical variables and model parameters which are difficult to measure and estimate. However, with good data, a skilled modeler can use a calibrated sediment model to predict regional, long term trends that can inform planning decisions and evaluate project alternatives. HEC-RAS includes mobile boundary, sediment transport modeling capabilities which route sediment and adjust channel cross sections in response to sediment dynamics. This chapter describes the theory and assumptions used for this analysis.