Code coverage records whether or not each line of code in a package is executed by the package's tests. While it does not check whether a given program or test executes properly it does reveal areas of the code which are untested. Coverage has a long history in the computer science community (Miller and Maloney in Communications of the ACM, 1963), unfortunately the R language has lacked a comprehensive and easy to use code coverage tool. The covr package was written to make it simple to measure and report test coverage for R, C, C++ and Fortran code in R packages. It has measurably improved testing for numerous packages and also serves as an informative indicator of package reliability. Covr is now used routinely by over 1000 packages on CRAN, Bioconductor and GitHub. I will discuss how covr works, how it is best used and how it has demonstrably improved test coverage in R packages since its release.