An account is given of some recent work on the mode theory of VLF ionospheric propagation. Attention is confined to the behavior of the attenuation coefficient of the dominant mode. The ionosphere is assumed to be a sharply bounded and homogeneous ionized medium. It is indicated that earth curvature increases the attenuation rate by as much as a factor of 2 as compared with the corresponding attenuation for a flat earth. The influence of the earth's magnetic field is also shown to be important. In fact, east-to-west propagation paths suffer much greater attenuation than west-to-east paths. The theoretical results in the present paper appear to agree well with the experimental data of W.L. Taylor.