Weather-resistive barriers (WRBs) are typically used in exterior walls of low rise frame buildings under claddings such as stucco (cement plaster), wood and wood derived products, vinyl (PVC), and masonry veneer. WRBs are primarily intended to provide resistance to water that may penetrate the outer cladding. WRBs also provide resistance to passage of air to varying extents but generally are moderately permeable to water vapor. Traditional WRB materials were limited to asphalt saturated felts and papers, but polymeric sheets have taken a growing share of the market in recent years. There are also trowel-applied and rigid board WRBs. Little information is available about the comparable properties of commercially available materials or what to consider when selecting the appropriate product for a particular application. Both building code requirements and vendors' product information are inconsistent and confusing. This paper, which is limited to the properties of water resistance and water vapor permeance, provides information that may be helpful in understanding, selecting, and using weather resistive barriers.