This document discusses various radio propagation models, their purposes and limitations. It describes common propagation modes like ground wave, diffraction, reflections and scattering. Popular models mentioned include ITU recommendations, Okumura-Hata, Longley-Rice and NRAO TAP which account for terrain, buildings, precipitation and other factors to predict transmission loss over different frequencies and path lengths. However, all models have limitations and may give different results so care must be taken in choosing the right model and interpreting predictions statistically.