Advances in our fundamental understanding of and control over interparticle colloidal forces have enhanced our ability to formulate stable, complex fluids with colloidal and/or nano-sized particles with specific rheological properties. This understanding stems from advances in experimental methods that probe these forces, either directly or indirectly, as well as theoretical treatments and simulation methods linking macroscopic suspension properties to the dynamics and interactions between colloidal particles. This article highlights recent experimental developments in the structure—property relationships for colloidal dispersions, with emphasis on the sensitivity of colloid rheology to nanometer-scale interactions. Examples of applications are used to illustrate these relationships.