Understanding colloid transport in porous media under transient-flow conditions is crucial in understanding contaminant transport in soil or the vadose zone where flow conditions vary constantly. In this article, we provide a review of experimental studies, numerical approaches, and new technologies available to determine the transport of colloids in transient flow. Experiments indicate that soil structure and preferential flow are primary factors. In undisturbed soils with preferential flow pathways, macropores serve as main conduits for colloid transport. In homogeneously packed soil, the soil matrix often serves as filter. At the macroscale, transient flow facilitates colloid transport by frequently disturbing the force balance that retains colloids in the soil as indicated by the offset between colloid breakthrough peaks and discharge peaks. At the pore-scale and under saturated condition, straining, and attachment at solid–water interfaces are the main mechanisms for colloid retention. Variably saturated conditions add more complexity, such as immobile water zones, film straining, attachment to air–water interfaces, and air–water–solid contact lines. Filter ripening, size exclusion, ionic strength, and hydrophobicity are identified as the most influential factors. Our review indicates that microscale and continuum-scale models for colloid transport under transient-flow conditions are rare, compared to the numerous steady-state models. The few transient flow models that do exist are highly parameterized and suffer from a lack of a priori information of required pore-scale parameters. However, new techniques are becoming available to measure colloid transport in real-time and in a nondestructive way that might help to better understand transient flow colloid transport. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality.