Interfacial tension controls many important nanoparticle physicochemical properties. As the particle diameter decreases the vapor pressure over a curved particle increases and the melting point decreases. This, in turn, influences phase transitions (nucleation of new particles, deliquescence and efflorescence, ice nucleation, and cloud droplet nucleation), the degree to which particles take up water at elevated relative humidity, and the tendency of particles to evaporate. Current theories are based on Gibbs’ thermodynamics, which yield the Kelvin equation and Gibbs-Thomson equation.