Atmospheric aerosols impair visibility and human health, interfere with radiative transfer, and alter cloud formation. The major contributors include sulfate and organic aerosols from anthropogenic and biogenic activities, which are produced through a multitude of complex multiphase atmospheric processes by photochemical oxidation of emitted sulfur dioxide and volatile organic compounds (VOCs) into less volatile forms and gas-to-particle conversion. Aerosol nucleation events have been frequently observed under various tropospheric conditions and account for a major fraction of the total aerosol population, but the fundamental chemical processes responsible for aerosol nucleation and growth remain poorly understood. New particle formation occurs in two distinct stages, nucleation to form thermodynamically stable clusters and subsequent growth of the newly nucleated clusters by condensation or heterogeneous reactions to detectable nano-sized particles. Sulfuric acid has been identified as a key species in new particle formation, but it is commonly recognized that binary nucleation of water and sulfuric acid is not efficient enough to explain measured nucleation events. Considerable uncertainty exists regarding the identity of other condensable species responsible for nucleation and growth of new particles. This talk will focus on nucleation and growth of nano-sized particles in the atmosphere. Experimental and theoretical results on this subject will be presented.