Attic Radiant Barriers (RBs) are proven technologies that significantly reduce the flow of radiant heat across attic spaces. This decreases the heat flow across the ceilings of buildings, which in turn lowers space cooling and heating loads, and produces energy and cost savings. This paper provides a general description of RBs, including installation configurations, the physical principles that make them work, and the laboratory and field experiments used to evaluate their thermal performance. An extensive review of the literature is summarized, highlighting fundamental issues, such as reduced ceiling heat flows, reduced space cooling and heating loads, and changes in attic temperatures produced by the installation of RBs in residential attics. Causes that affect RB performance, such as the influence of attic insulation level and climate, are presented. The data indicate that, on average, RBs reduce summer ceiling heat flows by 23 to 45% depending on the insulation level, whereas winter ceiling heat flow reductions are about 40% of the summer values for the same insulation levels. The data also indicate that RBs reduce space cooling loads by 6 to 20% and that space heating loads reductions arealso about 40% of the space cooling load values for the same insulation levels.