The distribution of colloid-associated contaminants and colloid-sized microorganisms within the vadose zone is controlled, in part, by mobilization and retention processes. In this work, we present pore-scale observations of colloid mobilization, transport, and retention within a transparent flow cell packed with partially saturated quartz sand. Our results show that colloid retention is dominated by adhesion to insular air bubbles at high moisture content and a combination of thin-film straining and immobile water storage at low moisture content. Mobilization of retained colloids under steady flow is governed by slow exchange between zones of immobile and mobile water. During transient flow, characterized by increases in flow rate and moisture content, strained colloids are released from expanding thin films of water and from immobile water zones that reconnect with advecting pore water regions. On the basis of our observations we infer that colloid mobilization and retention within the vadose zone reflects contributions from multiple mechanisms that are sensitive to moisture content and flow regime.