Colloidal contaminants such as pathogenic microorganisms and engineered nanoparticles enter subsurface from various sources such as land application of wastewater, reuse of untreated sewage for irrigation and sanitary landfills. Understanding colloid transport in the subsurface is essential for assessing the safe distance of drinking water wells from the source of contamination, bioremediation of contaminated sites, and degree of treatment required for land application of wastewater. There is a large disparity in the length scales associated with colloid transport in soil. This includes micrometre, centimetre, tens of centimetres, metre and kilometre scales which are representative of a single soil capillary, representative elementary volume scale, 1D lab scale, 3D lab scale, and the field scale, respectively. Colloid deposition mechanisms are scale dependent and are governed by the heterogeneity at that scale. Hence, the observed transport processes and the estimated parameters at a smaller scale may not simulate the observations at a larger scale. It is imperative to link the processes and the associated parameters across scales to better predict transport behavior at larger scales. This chapter discusses the mechanisms of colloid retention in porous media at various scales, the effect of heterogeneity on colloid transport at each scale, and upscaling of transport processes. © 2022, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.