Abstract: A timer for use during an interval training workout or other practice to measure and to display in countdown form the amount of time remaining in each of a series of sets of exercise periods, each exercise period followed by a rest period, and each exercise period and each rest period having a respective predetermined length. Provision is made for entering the length of the exercise period and the rest period for each set and for timing similar sets of exercise and rest periods a predetermined number of times. A countdown display shows the number of sets of exercise and rest periods remaining to be timed during a programmed workout, and an audible indication is provided at the end of each exercise period and at the end of each rest period. Provision is made to repeat a workout a single time or to repeat continuously the sequence of exercise and rest periods of a workout or practice session.