It would be a great accomplishment if we could simply design and build devices without defects. That should always be our goal—but, obtaining perfection is indeed a difficult/impossible challenge. Given that all devices will likely have a small fraction of the population which is defective, the question that we want to address in this chapter is: can a relatively short-duration stress be used to eliminate the defective/weak devices without causing significant degradation to the good/ strong devices? The use of a short-duration stress to eliminate weak devices is generally referred to as screening. We will find that screening can sometimes be very effective, but not always. Screening effectiveness depends on the exact details of the strength distribution for the devices plus the magnitude and duration of the screening stress.