Code coverage is often defined as a measure of the degree to which the source code of a program has been tested . Various metrics for measuring code coverage exist. The vast majority of these metrics require instrumenting the source code to produce coverage data. However, for certain coverage metrics, it is also possible to instrument object code to produce coverage data. Traditionally, such instrumentation has been considered inferior to source level instrumentation because source code is the focus of code coverage. Our experience shows that object code instrumentation, specifically post-link instrumentation, can be very useful to users. Moreover, it does not only alleviate certain side-effects of source-level instrumentation, especially those related to compiler optimizations, but also lends itself to performance optimization that enables low-overhead instrumentation. Our experiments show an average of less than 1% overhead for instrumentation at the function level and an average of 4.1% and 0.4% overhead for SPECint2000 and SPECfp2000, respectively, for instrumentation at the basic block level. This paper demonstrates the advantages of post-link coverage and describes effective methodology and technology for applying it.