This is the only chapter of the book devoted to the characterizations and measurements of the nanoparticles themselves, although it does include material related to the interaction forces between nanoparticles and cells. Zeta potential is probably the most important factor in determining whether nanoparticles will agglomerate in clusters. If you observe agglomeration happening to your nanoparticles (a very common problem), it is likely to be a problem with the zeta potential of your nanoparticles. The focus of the chapter is on the importance of “zeta potential,” which governs the fundamental electrostatic interactions of nanoparticles with each other as well as nanoparticle interactions with cells in an aqueous environment. Zeta potential is perhaps the single most important design consideration of nanomedical systems.