Secondary aerosol particles originating from vapors make a substantial contribution to the particle number, thus affecting the dynamics and properties of aerosol distributions, and consequently aerosol impacts on clouds and climate. This seminar discusses recent studies conducted together with e.g. University of Helsinki, Peking University and University of California, Irvine, on the very initial steps of new particle formation occurring at sizes below a couple of nanometers, and the effects of the process in different atmospheric environments. The focus is on sulfuric acid and base species – for reasons to be revealed in the seminar. Finally, the current best understanding and the most critical knowledge gaps concerning initial nanoparticle formation processes are summarized.