Colloids are defined as particles ranging in size from submicron to a few micron; their transport in the subsurface is of concern to drinking water quality due to introduction of viruses, pathogenenic bacteria and protozoans, as well as the potential for co-transport of toxic chemicals sorbed to mobile mineral colloids. More than 2000 published papers have addressed the topic of colloid transport, but the vast majority of these studies focused on water-saturated (groundwater) environments, even though most pathogens and toxicants enter groundwater via transport through the shallower soil which is only partially water-saturated. Further, almost all of studies of unsaturated systems are limited to steady state flow, while in nature, flow in partially saturated porous media is dominated by transient wetting events (e.g., storms, or flushing toilets). Early work attributed colloid retention under partial saturation to attachment to the air-water interface or “staining” in thin water films. More recent work, including collaborative studies between the University of Tennessee and Cornell University has called this prevailing paradigm into question. The work presented here uses a novel experimental approach to evaluate colloid transport under transient wetting fronts, and evaluates the relative importance of water content, colloid size and surface charge, and the role of electrostatic and capillary forces in colloid immobilization. We also compare transport in natural porous material (sand) and uniform model silica-sphere medium; the silica-sphere medium has a known pore structure, which will facilitate efforts to develop mechanistic transport models. Results are expected to improve predictions of pathogen transport in different geological settings and natural flow conditions, and may suggest novel strategies to mitigate human and environmental health risks in public water supplies.