This book presents an overview of the theory of mixed models applied to problems in medical research. It is easily accessible to practitioners in any area where mixed models are used, including medical statisticians and economists; includes numerous examples using real data from medical and health research, and epidemiology, illustrated with SAS code and output; and features new version of SAS, including the procedure PROC GLIMMIX and an introduction to other available software. This second edition will be useful for applied statisticians working in medical research and the pharmaceutical industry, as well as teachers and students of statistics courses in mixed models. The text will also be of great value to a broad range of scientists, particularly those working the medical and pharmaceutical areas.