This chapter describes a method for the measurement of the electrostatic potential at the electrical double layer surrounding a nanoparticle in solution. This is referred to as the zeta potential. Nanoparticles with a zeta potential between −10 and +10 mV are considered approximately neutral, while nanoparticles with zeta potentials of greater than +30 mV or less than −30 mV are considered strongly cationic and strongly anionic, respectively. Since most cellular membranes are negatively charged, zeta potential can affect a nanoparticle’s tendency to permeate membranes, with cationic particles generally displaying more toxicity associated with cell wall disruption. This technique is demonstrated for two types of nanoparticles commonly used in biological applications: colloidal gold (strongly anionic) and amine-terminated PAMAM dendrimer (strongly cationic).