The advancement of computer systems has led to the existence of large and detailed existing computerized data sets, many of which contain useful information for studying the elderly and the aging process. This paper identifies several types of existing large data bases and provides examples of each. Strengths and weaknesses of working with existing data are discussed and strategies for conducting secondary data analysis of large data sets are outlined. Existing data sets represent large investments in the collection of individual data and have great potential to provide answers to important questions in the field of aging research.