The competitive nature of business today, coupled with incentives by utility companies is encouraging users to conserve energy by varying the speed of cooling tower fans in place of more traditional across the line cycling. There are several options available to exploit the benefits of the relationship between fan speed, cooling tower performance and fan horsepower. Variable frequency drives (VFD) are becoming the most popular alternative for controlling tower capacity by varying the fan speed, especially considering the technological advances in VFD's during the last two decades. A challenge in successful operation of cooling towers today with VFD's revolves more around proper application than technological limitations in the equipment. A concise discussion of the do's and don'ts of VFD's will be presented.