To support the understanding, preparation, and long-term development of youth rugby players, the accurate measurement of their physical qualities is vital. This chapter summarises how anthropometry, body composition, strength, power, speed, agility and change-of-direction, aerobic and anaerobic capacity, and athletic movement skills are measured within youth rugby players and discusses the accuracy and reliability of these methods. Furthermore, the implications of using these different testing methods within research are considered. Due to the large discrepancies in testing outcomes between rugby players of similar ages, this chapter will provide recommendations for accurate and reproducible testing of youth rugby players. Additionally, future research directions are provided that will enhance the understanding of youth rugby player development.