With today's high technology and highly reliable products, the traditional life tests result in none or very few failures by the end of the test. The life test is, therefore, fully or partially accelerated to induce early failures and obtain quickly information about the life distribution of the product. The test data obtained at accelerated conditions are extrapolated by means of an appropriate model to the use conditions to quantify the life characteristics of the product under normal use conditions. In fully accelerated life testing all the test units are run at accelerated condition, while in partially accelerated life testing they are run at both normal and accelerated conditions. Stress under accelerated condition can be applied using constant-stress, step-stress, progressive-stress, cyclic-stress, random-stress, or combinations of such loadings. The choice of a stress loading depends on how the product or unit is used in service and other practical and theoretical limitations.