ASHRAE Technical Committee (TC) 9.9 “Mission Critical Facilities, Technology Spaces and Electronic Equipment” was established to create guidance on the environmental protection of such environments in an energy conscious manner. It’s extremely important to note the first word in the quotes of the preceding sentence – Mission. Mission must always precede energy in terms of priorities. Energy consciousness does not mean that energy savings should be pursued at the expense of mission. Unfortunately, understanding where this line is drawn is not very easy and it takes delving into many finer details to distinguish the difference. One particular area of great energy savings, but equally great challenges to the mission, is the use of economizers. This paper attempts to arm the reader with an increased understanding of several aspects of the application of both airside and waterside economizers to datacom facilities so that the right decisions and considerations can be made to serve BOTH mission and energy.