IEEE Std 1564-2014 Guide for Voltage Sag Indices is a new standard that identifies appropriate voltage sag indices and characteristics of electrical power and supply systems as well as the methods for their calculation. This paper presents an overview of IEEE 1564, which was approved as a new standard by IEEE RevCom in March 2014. The paper summarizes the IEEE 1564 methods for quantifying the severity of individual voltage sag events, for quantifying the performance at a specific location via single-site indices, and for quantifying the performance via system indices. Multiple methods are presented for each. The methods are appropriate for use in transmission, distribution, and utilization electric power systems.