The 'molecular' aspect of molecular epidemiology refers to the application of molecular biological approaches to epidemiologic problems. The variety and power of molecular methods are rapidly expanding our capability to dissect etiologic mechanisms of disease. The 'epidemiologic' aspect,meanwhile, refers to utilizing the tools and perspectives of epidemiology to comprehend observations in molecular biology. An increasingly sophisticated variety of molecular techniques are being applied to generate an explosion of data on exposures and outcomes whose significance must be determined.The wealth of information generated by the Human Genome Project and the corresponding efforts in microbial genomics are providing researchers with extraordinary new opportunities to further understand the etiology of disease. Molecular epidemiology is rapidly evolving through the application of morerelevant markers of exposure and better identification of subgroups at increased risk of early disease processes and progression.This volume provides a broad overview of the diverse approaches encompassed by this emerging discipline. Chapters 1-3 highlight concepts and methods at the forefront of studies of infectious diseases, and include a synopsis of general epidemiologic design and analysis, as well as current techniquesfor molecular typing of microbial pathogens. Chapters 4-6 are focussed on recent advances in studying host genetics, including methods for the detection of mutations and polymorphisms, identification of linkage among alleles of genes on a given chromosome, and the use of DNA pooling to increaseefficiency in association studies of genetically complex traits. Chapters 7 and 8 deal with the relationship between pathologies and immune system genetics, and on the use of DNA sequence information to garner insights into the evolution of infectious pathogens, and into the evolution and functionof the immune system.