High altitude cirrus clouds influence the climate by scattering and absorption of radiation. The cloud and aerosol chamber AIDA of Forschungszentrum Karlsruhe allows generation and observation of ice clouds in laboratory at realistic conditions. Experiments were carried out to investigate the ice formation in clouds of supercooled liquid water droplets and in sulphate aerosols. The homogeneous ice nucleation rate given as the product of the number of critical ice germs to initiate the freezing and the rate at which additional molecules are incorporated into a critical germ, was measured at typical cloud conditions in the temperature range between −36 °C and −37 °C for supercooled water droplets with diameters smaller than 10 µm. The comparison of the results shows good agreement both with recent data from literature gained from considerably larger droplets and with classical ice nucleation theory. Thereby, the hypothesis that a critical germ is formed preferentially near the surface of a supercooled droplet could not be confirmed. The ice formation from ammonium sulphate aerosol was examined in the temperature range from −50 °C to −65 °C. It was shown that ammonium sulphate solution particles, which were produced by neutralization of sulphuric acid solution particles by gaseous ammonia, freeze homogeneously at conditions comparable to sulphuric acid solution particles. If ammonium sulphate particles crystallize they start to form ice at comparatively low supersaturation values (10–20%) by direct deposition of water vapour onto the solid particles. Ammonia, which is highly anthropogenically affected, may influence the properties of upper tropospheric ice clouds.