Integral systems test (IST) facilities, which are sometimes referred to as integral effects tests (IETs), play a key role in the design, assessment, and certification of innovative reactor designs. Data obtained using such facilities have been used to benchmark the best-estimate safety analysis computer codes used to evaluate nuclear plant safety. They have also been used to assess the effectiveness of safety system functions under simulated accident conditions. Scaling analyses are an important component in determining the applicability of an evaluation model for its intended purpose. Evaluation models can only approximate the physical behavior of postulated events on a prototype, and the judgment of their adequacy relies heavily on their capability in predicting IETs that were designed to simulate the events analyzed. With few exceptions, all ISTs are subscale representations of the power plants they are built to represent. As thermal-hydraulic computer codes are benchmarked to an IST database, the most important challenge for the analyst is the development of convincing scaling similarities between the IST and the plant. Unless such rules are clearly identified, it would not be possible to extend conclusions from the assessment of the tools against subscale data to the plant scenario time and geometry scales.