The purpose of this chapter is to introduce the basic principles of epidemiology, the methods used, and the application of these methods to a practical field setting. Although the discussion focuses on the practice of epidemiology in relation to arbovirus diseases, the principles and methods apply equally to other infectious diseases and noninfectious diseases as well. Epidemiologic analyses range from the most simple, almost intuitive, procedures to highly complex functions requiring extensive knowledge of biostatistics as well. The thrust of this chapter is on the practical aspects of an epidemiologic investigation of arbovirus disease emphasizing surveillance techniques, investigative procedures in the field, and appropriate analytic methods. The practicality of using mortality statistics as an assessment of arbovirus diseases will vary tremendously by time, location, and disease. For many infectious diseases, particularly bacterial, confirmation of the disease in question can be relatively simple and rapid. With other diseases, the clinical syndrome is virtually pathognomonic, such as measles and chicken pox.