Epidemiology is the basic medical science that focuses on the distribution and determinants of disease frequency in human populations. An understanding of the tools of epidemiology is helpful in defining the limitations of medical research and evaluating the conclusions of studies. This is the first in a series of three articles whose objective will be to present the basic concepts of epidemiology and biostatistics. Examples of each of the tools and limitations discussed from studies of patients with end-stage renal disease (ESRD) will be presented to provide the reader with a practical application of the concepts. This series of articles will help the reader to weigh methods and study designs to understand the appropriate conclusions that may be drawn from any data.