In the past, IT equipment manufacturers and their data center operators were focused more on maximum performance, not energy efficiency. While the newest generations of computing hardware have continued to increase in performance, they have also become highly focused on energy efficiency. In fact it has been shown that in many cases the cost of energy for an older commodity server is higher over a three year period than the cost of the server itself. This is especially true in older data centers when the cost energy for the supporting power and cooling infrastruc¬ture is added to the total energy cost (See Understanding PUE in the facilities section of the DCK Guide to Energy Efficiency, page 5). This article is an abridged version of the IT System Efficiency section of the DCK Executive Guide to Energy Efficiency and the second article in a 5 part series on Energy Efficiency.