For the design of embedded systems that support real-time applications, it is required to guarantee the satisfaction of real-time constraints in any case. For an embedded application that runs repeatedly, the real-time constraint is usually defined by the relative deadline that is the maximum allowable execution delay of an application after being released. The response time of an application is defined as the time interval from the release time to the finish time. After applications are mapped to a candidate architecture, we check the feasibility of the architecture by estimating the worst case response time (WCRT) of an application. Fast estimation enables us to explore the wider design space of architecture selection and application mapping. Accurate estimation avoids over-design to reduce the system cost. However, estimation speed and accuracy are in general incompatible since the accuracy induces higher complexity. Hence various performance estimation techniques are required in the design process.