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With clock rates beyond 1 GHz, the model of a system wide synchronous clock is becoming difficult to maintain; therefore, asynchronous design styles are increasingly receiving attention. While the traditional synchronous design style is well-proven and backed up by a rich field experience, comparatively little is known about the properties of asynchronous circuits in practical application. In the face of increased transient fault rates, robustness is a crucial property, and from a conceptual view, the so-called ldquodelay-insensitiverdquo asynchronous design approaches promise to be more robust than synchronous ones, since their operation does not depend on tight timing margins, and data are two-rail coded. A practical assessment of asynchronous designs in fault-injection (FI) studies, however, can rarely be found, and there is a lack of adequate methods and tools in this particular domain. Therefore, the objective of this work is 1) to provide a common approach for efficient and accurate FI in synchronous and in asynchronous designs, and 2) to experimentally compare the robustness of both synchronous and asynchronous designs. To this end, a synchronous 16-bit processor as well as its asynchronous (delay insensitive) equivalent are subjected to signal flips and delay faults. The results of over 489 million experiments are summarized and discussed, and a detailed discussion on the specific properties of the chosen asynchronous design style is given.