Inquiring customers want to know: Why is the temperature monitoring system sensor showing a different number than the thermostat? Is it broken, mis-calibrated, inaccurate? Maybe, but not likely. Using the example of a room in a home, the paper considers the significant factors that can result in different values. Specific considerations for critical applications such as server, computer and telecommunication rooms, data centers, walk-in refrigerators and freezers are also discussed.