We investigate the physical and chemical properties of the stratospheric clouds produced by the recent eruptions of the Mount St. Helens volcano. The large and diverse set of observational data collected in the high-altitude plumes of the May 18, May 25, and June 13, 1980 eruptions is organized and analyzed for evidence of the processes at work. The data are used to guide and constrain detailed model simulations of the volcanic clouds. For this purpose a comprehensive one-dimensional model of stratospheric sulfate aerosols, sulfur precursor gases, and volcanic ash and dust is utilized. The model accounts for gas-phase and condensed-phase (heterogeneous) chemistry in the clouds, aerosol nucleation and growth, and cloud expansion. Computational results are given for the time histories of the gaseous species concentrations, aerosol size distributions, and ash burdens of the eruption clouds. The long-term buildup of stratospheric aerosols in the Northern Hemisphere, and the persistent effects of injected chlorine and water vapor on stratospheric ozone, are also investigated. We conclude that SO2, water vapor, and ash were probably the most important substances injected into the stratosphere by the Mount St. Helens volcano, both with respect to their widespread effects on composition and their impact on climate. Despite the seemingly large magnitude of the Mt. St. Helens eruption, we also find that the volcano had little influence on the climate (≲0.05 K surface cooling in the Northern Hemisphere) or on stratospheric ozone (<0.2% maximum hemispherical depletion). The remaining problems in the field of volcanic cloud physics and chemistry, and the requirements for future experimental work, are also discussed.