The subject of early data on mortality is a vast one, and thus this treatment is quite broad. The emphasis is on identifying classes of data, sources of ambiguity, and general approaches to problems of interpretation. Statistics collected and special studies prepared by life-insurance companies represent an underutilized body of data for nineteenth-century mortality in the United States and Europe. There are two broad approaches to estimating completeness of death registration: direct and indirect. Relatively few sources of early mortality statistics can satisfy the Preston-Keyfitz-Schoen criteria. The deficiencies of such data are definition of death, misallocation of deaths by place of occurrence, age misreporting, completeness of registration, and cause misreporting. Errors in ascertaining true cause of death may arise from inaccurate diagnosis by the attending physician, failure to perform an autopsy, inaccurate autopsy results, including inability of the pathologist to specify multiple causes of death, and failure of the death certifier to take into account the results of autopsy.