A review is provided of the reasons why the U.S. Nuclear Regulatory Commission needs thermal-hydraulic system computer codes, the assumptions and approximations contained within these codes, and the reasons why test data are required to assess the accuracy of the codes. Specific examples of codes and test programs are given. The use of computer codes assessed against data from scaled test facilities to predict the full-scale plant response is discussed. A method to help focus resources and the need for quantifying code uncertainties are discussed.