There is a growing awareness, both in industry and academia, of the crucial role of formally verifying the translation from high-level source-code into low-level object code that is typically performed by an optimizing compiler. Formally verifying an optimizing compiler, as one would verify any other large program, is not feasible due to its size, ongoing evolution and modification, and, possibly, proprietary considerations. Translation validation is a novel approach that offers an alternative to the verification of translators in general and compilers in particular: Rather than verifying the compiler itself, one constructs a validation tool which, after every run of the compiler, formally confirms that the target code produced in the run is a correct translation of the source program. The paper presents voc, a methodology for the translation validation of optimizing compilers. We distinguish between structure preserving optimizations, for which we establish a simulation relation between the source and target code based on computational induction, and structure modifying optimizations, for which we develop specialized permutation rules. The paper also describes voc-64 - a prototype translation validator tool that automatically produces verification conditions for the global optimizations of the SGI Pro-64 compiler.