The cost of silicon debug can be considerable and unpredictable. Problems can range from catastrophic to subtle. Once errors have been observed, debug takes over. When problems arise, the first challenge is to categorize them. Causes can range from incorrect specifications to silicon defects, to measurement errors. This special issue focuses on all aspects of a successful debug process: how to prepare, what to do during debug, and how to use the results to improve things in the future. The seven articles in this issue cover a broad variety of topics in silicon debug and diagnosis, as well as the newly emerging middle ground between the two: at-speed timing failures.