  As we move to distribute High Energy Physics computing tasks throughout the
global Grid, we are encountering ever more severe difficulties installing and
selecting appropriate versions of the supporting products. Problems show up at
every level: the base operating systems and tools, general purpose utilities
like root, and specific releases of application code. I will discuss some
specific examples, including what we've learned in commissioning the SAM
software for use by CDF. I will show why revision control can be a truly
difficult problem, and will discuss what we've been doing to measure and
control the situation.
