Long-term exposures to toxic chemicals can pose risks to human health. In order to properly assess and minimize such risks, it is necessary to measure chemical levels in the air or other environmental media. Since exposure levels vary across groups, between persons (in a given group), and within persons (over time), appropriate statistical analysis of exposure data typically requires the use of linear mixed effects models. This book applies such statistical models to exposure data, with emphasis upon airborne chemical exposures in the workplace and in the ambient environment.