A first report is presented on a comprehensive study of stratospheric aerosols up to 30 km altitude. Information was obtained on the vertical profiles as a function of particle size, on the size distribution of these particles and on their physical structure and chemical composition. Vertical concentration profiles are first considered theoretically on the basis of equilibria between eddy diffusion, sedimentation, subsidence and coagulation. The results are compared with two sets of data: (1) vertical profiles obtained with a recording Aitken nuclei counter which measures the size range of 0.01 to 0.1 microns radius and (2) vertical profiles obtained with impactors over the size range of 0.1 to 1.0 microns radius. The first of these indicates a decrease in concentration from the tropopause to nearly zero at 20 km, whereas the second shows a maximum at about 20 km. The size distributions of these particles were found to be quite constant with time. There is a maximum between 0.01 and 0.1 μ radius, the concentration varying inversely with the square of the radius between 0.1 and 1.0 μ. Very few data could be obtained for particles larger than 1.0 μ because of the low concentration. Large fluctuations seem to be indicated for this size range. The particles between 0.1 and 1.0 μ were collected in sufficient concentration to permit detailed investigation. They were found to be very hygroscopic and, by electron microprobe analysis, were found to contain sulfur as a major constituent, with traces of iron and silicon. From these observations, it is tentatively concluded that there are three major populations of particles present in the stratosphere. Those smaller than 0.1 μ are of tropospheric origin. Those between 0.1 and 1.0 μ are most likely formed within the stratosphere, possibly by oxidation of SO2 and H2S gas traces. Fall speed considerations make it very likely that the majority of particles larger than one micron are of extra-terrestrial origin.