For more than two centuries, scientists have been puzzled by observations of solar-climate variability yet the lack of any established physical mechanism. Some recent observations, although disputed, suggest that clouds may be influenced by cosmic rays, which are modulated by the solar wind. The CLOUD experiment aims to settle the question of whether or not cosmic rays have a climatically-significant effect on clouds by carrying out a series of carefully-controlled measurements in a large cloud chamber exposed to an ionising particle beam from the CERN Proton Synchrotron. By current estimates, about half of all cloud droplets are formed on aerosol particles that were “nucleated” from the clustering of trace atmospheric vapours. This talk will present the scientific motivation for CLOUD and the first results on the influence of cosmic ray ionisation on the nucleation and growth of atmospheric aerosol particles.