Nano-emulsions are emulsions with droplet size in the nanometer scale (typically in the range 20-200 nm). Due to the extremely small droplet size, nano-emulsions appear transparent or translucent (resembling microemulsions) and possess stability against sedimentation or creaming. The formation of kinetically stable liquid/liquid dispersions of such small sizes is of great interest from fundamental and applied viewpoints. Nano-emulsions are generally prepared by dispersion or high-energy emulsification methods. However, condensation or low-energy methods, which make use of the phase transitions taking place during the emulsification process, are receiving an increasing attention in this context. This communication will focus on the potential of surfactant phase behavior information for the preparation of controlled size nano-emulsions by low-energy methods. Phase behavior studies in water/polyethoxylated nonionic surfactant/oil systems have shown that phase transitions during the emulsification process involving lamellar liquid crystalline and/or bicontinuous microemulsions phases are required to achieve minimum droplet size. Examples will be given on the formation of O/W nano- emulsions with droplet sizes as low as 20 nm, low polydispersity and high kinetic stability. In addition, the use of nano-emulsion droplets as templates for the preparation of nanoparticles will be also described.