This chapter shows how to calculate unsteady flow water surface profiles using the 1D Unsteady Flow solver in HEC-RAS. If the user is interested in 2D modeling, or combined 1D/2D, please review the 2D Modeling User's Manual. The chapter is divided into four parts. The first part explains how to enter unsteady flow data and boundary conditions. The second part describes how to develop a plan and perform the calculations. The third part focuses on calibrating an unsteady flow model, and the last part talks about model accuracy, stability, and sensitivity.