▪ Abstract The interaction of a flexible structure with a flowing fluid in which it is submersed or by which it is surrounded gives rise to a rich variety of physical phenomena with applications in many fields of engineering, for example, the stability and response of aircraft wings, the flow of blood through arteries, the response of bridges and tall buildings to winds, the vibration of turbine and compressor blades, and the oscillation of heat exchangers. To understand these phenomena we need to model both the structure and the fluid. However, in keeping with the overall theme of this volume, the emphasis here is on the fluid models. Also, the applications are largely drawn from aerospace engineering although the methods and fundamental physical phenomena have much wider applications. In the present article, we emphasize recent developments and future challenges. To provide a context for these, the article begins with a description of the various physical models for a fluid undergoing time-dependent motion, then moves to a discussion of the distinction between linear and nonlinear models, time-linearized models and their solution in either the time or frequency domains, and various methods for treating nonlinear models, including time marching, harmonic balance, and systems identification. We conclude with an extended treatment of the modal character of time-dependent flows and the construction of reduced-order models based on an expansion in terms of fluid modes. The emphasis is on the enhanced physical understanding and dramatic reductions in computational cost made possible by reduced-order models, time linearization, and methodologies drawn from dynamical system theory.