Cosmic rays generate molecular cluster ions throughout the atmosphere, and are the principal source of lower atmosphere ionisation. Cluster ions cause the electrical conductivity of atmospheric air, which permits currents to flow continuously from disturbed weather (thunderstorm) areas to fair weather regions. It has been suggested that cosmic ray changes are linked with cloud, through the influence of atmospheric cluster ions. As clouds have an appreciable influence on climate, the global nature of cosmic ray ionisation could result in climate effects. Two physical processes identified as important are: (1) production of ultra-fine aerosol particles by high energy ionisation and (2) the effect of aerosol particle and droplet charging at cloud boundaries. Mechanisms have been theoretically established to support both these processes, and evidence from experimental data is becoming available. This talk will consider effects of cosmic ray ionisation on aerosols and clouds.