Abstract: Because the flow of gases and liquids is affected by and interacts with solids brought into the stream, invasive measurement techniques are not suitable for all application scenarios. Non-invasive techniques enable taking measurements from a distance but may measure only integrated quantities along rays of projection. Optical techniques, in particular, are constrained by the minimal interaction of light with most gases and liquids as well as the fact that the visual appearance of flow phenomena is only indirectly related to the physical quantities of density, pressure, and velocity that underlie flow behavior. On the other hand, image-based measurement approaches enable acquiring millions of data points simultaneously to sample complex flow fields densely in space and time. In my talk I will present several image-based techniques to capture and model surfaces, volumes, and velocity fields of different gaseous and liquid flow phenomena in nature. For most techniques, we rely on standard video cameras, and some approaches work with as few as a single camera. By making use of simple physical effects during acquisition like fluorescence or Schlieren in combination with state-of-the-art reconstruction algorithms including optical tomography, weighted minmal hypersurfaces, and compressed sensing, the geometry of complex flows can be modeled in 3D or even 4D.