The reliability of a system is defined as the probability that it will perform its required function under stated conditions for a stated period of time. Rapid changes in the technology, shorter periods in product development, and higher reliabilities of products make accelerated life tests even more useful and important in the industry of today. Accelerated life tests are used to obtain timely information on the times to failure distributions of components and systems. Test units are subjected to higher than the usual levels of stress. Then the test results are extrapolated from the test conditions to the usual use conditions via physically reasonable statistical models. Models based on Weibull statistics are widely used as accelerated-life-test models in reliability engineering. In this context, the aim of this section is to propose a new representation of the hot-carrier degradation in threshold voltage of MOS transistors by applying the Weibull distribution to the experimental observations and to predict the device behavior after a long stress time. The advantages provided by the method proposed is demonstrated on different application examples, namely on the properties of a current source-loaded single stage amplifier, on a CMOS inverter and on a CMOS OTA, operating in subthreshold region.