Observations of water vapor and cloud liquid made with surface-based microwave radiometers operating near 20, 23, 31, and 90 GHz are described. Measurements were made at several continental locations in the United States, an island and ship in the northern Atlantic Ocean, and an island and ship in the tropical western Pacific Ocean. The instrumentation and statistical inversion technique employed to retrieve vapor and liquid values from radiometric brightnesses are reviewed. Various aspects of the water vapor and cloud liquid data are compared as a function of geographic location. The diurnal and seasonal variability of water vapor, cloud liquid, cloud fraction, and infrared sky brightness temperature observed during a 1-year period at an island in the North Atlantic Ocean are also presented.