This paper presents a method for simulating the motion of water drops on a surface in real time. We describe the dynamics of a drop moving on the surface, and then we present our simulation model. We use a geometry-based representation of a drop. Each drop is modeled by a deformable 3D mesh. This geometrical representation allows drops to be on the surface or in the air. We also propose a simple method to handle drop merging and separation. For the rendering, we simulate reflection and refraction. The drop trace is also taken into account. Our method is fast and robust and yields realistic results when applied to treat condensation on a surface or human sweating in real time. Copyright © 2012 John Wiley & Sons, Ltd.