Fluid-fluid displacement in porous media occurs in many natural and industrial processes such as geological CO2 storage and enhanced oil recovery. It has been recognized that wettability plays an important role in the displacement process. Thanks to decades of research, we now have a good understanding of fluid-fluid displacement in porous media with uniform wettability. In contrast, our knowledge of fluid-fluid displacement in porous media with heterogeneous wettability (ie, mixed-wet) is much less complete, although mixed-wet states are common in many subsurface processes.