Should teachers teach the calculation of target heart rate to students? And when is it appropriate to engage students in the attainment of these heart rates during physical education class activities? The answers to these questions are not easy. One might be tempted to state a simple yes or no and to identify a specific age to begin using training heart-rate zones for exercise. To compound these issues, there is now a new formula to calculate heart rate max (HR max), and this new formula affects the calculation of training heart rates (Tanaka, Monahan, & Seals, 2000). The purpose of this article is to review current activity guidelines for children, provide basic information as to why it is inappropriate to use training heart-rate zones in prepubescent children, suggest a grade level when target heart-rate zones may be appropriate, and present the new HR max formula to calculate target heart rate.