Based on the working principle of thermosyphon, several thermodiode designs which allow heat to be transferred in the desired direction but block the heat flow in the opposite direction were developed and tested for solar heating applications. More recently thermodiodes with easily alterable direction of heat flow, so-called bi-directional thermodiodes, were invented and their performances tested and optimized for both summer-cooling and winter-heating of buildings and shelters. Presented in this paper are a review of single-phase thermodiodes applied to solar energy utilization and energy-efficient buildings.