Colloid-sized particles, including mineral fragments, plant detritus, and microbes, are ubiquitous in natural waters. The transport of these waterborne particles has important implications to ecological functioning, water quality, and geologic processes. In subsurface environments, particle transport influences the mobility of groundwater contaminants and controls soil-illuviation rates and soil-profile development. Particle transport in surface waters is equally important, affecting the stability of coastal marshes, the formation of landscape features of freshwater wetlands (e.g., tree islands), and nutrient cycling in streams and rivers. We have attempted to advance understanding of these issues by conducting laboratory and field studies intended to elucidate the mechanisms and physiochemical properties that govern the mobility of waterborne particles within natural environments. Our laboratory-based studies have centered on colloid transport and surface reactions within water-saturated and unsaturated sediments. Observations made during pore-scale visualization experiments reveal that colloid mobilization and deposition to air-water and solid-water interfaces is governed by multiple mechanisms, and findings from bench-top column experiments suggest that these mechanisms are sensitive to changes in volumetric moisture content, flow velocity, porewater composition, and colloid size, shape, and mineralogy. Results of other column experiments indicate that colloids mobilized from natural soils are capable of adsorbing and accelerating the transport of radionuclides (cesium and strontium) and herbicides (atrazine) during saturated and unsaturated flow through geologic materials. Our field experiments have been carried out in surface-water environments and have relied on particulate-tracer injections into streams that drain forested watersheds in New England and into surface-water wetlands of the Florida Everglades. Analyses of observations made in the stream-tracer experiments demonstrate that within-stream retention of micrometer-sized particles exhibits a strong seasonal dependence and that, for all seasons, the transport of these microscopic particles differs substantially from that of a conservative tracer. Particle transport within wetlands is complicated by the presence of emergent vegetation. Findings from our tracer experiments in the Everglades demonstrate that vegetation composition affects the advective-dispersive transport of bacteria-sized particles and that emergent plants serve as efficient collectors of these particles. We have used our data – from both the laboratory and field experiments – to test mathematical models for colloid transport and reaction in soils and in surface water. Although our attempts to simulate the observed phenomena cannot be considered a complete success, our findings suggests that suitable theoretical frameworks for particle transport in surface and subsurface environments are tractable.