Reference ranges are a set of values that correctly include most of the subjects with characteristics similar to the reference group and exclude the others. When accurate, reference ranges aid physicians to interpret results of clinical measurements and thus establish diagnosis. However, obtaining accurate reference ranges is a very demanding procedure. This chapter provides basic definitions and theories as well as a step-by-step procedure for the analysis of reference values and determination of reference ranges of coagulation, focusing on quantitative clinical laboratory assays. Preanalytical and analytical factors as well as dependence on the age influencing reference values for coagulation assays and their transference are discussed.