This chapter discusses the evolution of ideas underlying research and practice in medicine. An emergency medicine specialist does not always have the luxury of taking time out to reflect, as do internists or psychiatrists. Wherever good medicine is practiced, physicians go beyond the observation and analysis of a single case or event. To practice good medicine, generations of physicians have learned thousands of facts from books, lectures, and from personal communications with more experienced colleagues. The chapter defines at least some of the terms including epidemiology, clinical epidemiology, biostatistics, health economics, qualitative research and evidence-based medicine. Biostatistics is defined as the application of statistics to biological problems, which goes well beyond the field of medicine itself. Experience in the study of single individuals obtained from medicine, psychology and nursing was used, modified and expanded in social sciences, business, finances, administration, law and military.