The applications and theories of the insulated dipole and monopole are reviewed. A recent analysis of the insulated antenna in a relatively dense ambient medium is summarized and its predictions are shown to be in good agreement with an extensive new set of measurements of the distributions of current and charge and the driving‐point admittances of air‐insulated antennas in fresh and salt water. Four different thicknesses of insulation and lengths up to one wavelength are used. The attenuation due to radiation into fresh water is comparable with and even exceeds the attenuation due to dissipation in salt water. It increases with decreasing radius of the insulating cylinder. The effect of the junction between the feeding coaxial line and the insulated antenna is discussed.