The dielectric constants of silica aerogels are among the lowest measured for any solid material. The silica aerogels also exhibit low thermal expansion and are thermally stable to temperatures exceeding 500°C. However, due to the open porosity and large surface areas for aerogels, their dielectric constants are strongly affected by moisture and temperature. This paper presents data for the dielectric constants of silica aerogels as a function of moisture content at 25 °C, and as a function of temperature, for temperatures in the range from 25 °C to 450°C. Dielectric constant data are also given for silica aerogels that are heat treated in dry nitrogen at 500°C, then cooled to 25°C for measurements in dry air. All measurements are made on bulk aerogel spheres at 22GHz microwave frequency, using a cavity perturbation method. The results of the dependence found here for bulk materials can be inferred to apply also to thin films of silica aerogels having similar nano-structures and densities.