Data Centers are an ever increasing user of energy in our economy. While the performance per watt of our IT equipment continues to increase exponentially, this energy performance improvement is still outstripped by increasing demand. Because of this, the efficiency of data centers must continue to improve. Beyond just efficiency, many data centers now are working towards reuse of their waste energy in other areas in the data center or on the site or campus. How to account for this, through metrics and measurements, is the topic of this paper. The Energy Reuse Effectiveness metric or ERE is discussed; both the development and application of the metric are looked at in detail. The use of ERE in conjunction with PUE (Power Usage Effectiveness) is also considered.