This review focuses on the experimental determination and interpretation of capillary forces in the nanoparticle regime via atomic force microscopy (AFM), including an in-depth comparison of 28 AFM studies investigating the dependency of the capillary force between hydrophilic surfaces on the relative humidity of the system. Both colloidal probe experiments and experiments conducted with sharp AFM probes are considered. The dependence of the capillary forces on roughness, deformability, hydrophilicity, shape, and size, kinetics of meniscus formation and the relative humidity is discussed. These factors should be carefully considered in any experimental study in order to accurately determine the influence of capillary forces in nanoparticle adhesion.