Outdoor air infiltration through leakage pathways of the building envelope is an important contributor to building energy loads. The accurate determination of air inflow from a rigorous scientific standpoint is very complex due to temporal and spatial variability of the driving pressure differences across the building envelope and the practical difficulties of determining the location and size of envelope leakage pathways; simplifications are thus required for engineering purposes. We start by illustrating how infiltration affects building loads and by defining certain important terms. An overview of air infiltration in past and current building stock in the United States is then provided. Next, we present calculation methods to estimate the pressure difference across the envelope due to the natural driving forces of wind velocity and stack temperature difference. This is followed by a discussion of a widely used engineering model based on a simplification of the theoretical crack flow equation for determining air infiltration through actual building components and assemblies. We then provide a broad overview of multizone airflow models, followed by two commonly used simplified physical models for singlezone buildings along with illustrative examples. Due to the current interest in adopting hybrid ventilation as a strategy to lower energy use in a building, correlations to estimate natural ventilation caused by deliberate opening of windows and doors are also briefly covered. Finally, we describe different techniques, both in the laboratory and in the field, for measuring air infiltration and interzone flows.