This book is for the person who needs information about making time and frequency measurements. It is written at a level that will satisfy those with a casual interest as well as laboratory engineers and technicians who use time and frequency every day. It includes a brief discussion of time scales, discusses the roles of NIST and other national laboratories, and explains how time and frequency are internationally coordinated. It also describes the available time and frequency services and how to use them. It discusses the accuracies that can be achieved with the different services, and the pros and cons of using various calibration methods.