Agility and change of direction (COD) are distinct but crucial skills for team sport athletes. As athletes develop different performance aspects, the ability to control the body during COD is altered. Unplanned rapid COD, particularly during the deceleration phase, are the most likely cause of non-contact injuries in team sports. There is little previous research on deceleration to provide guidance for this component of performance, particularly with reference to rugby union. The aim of this thesis was to; 1) to determine if a period of deceleration training would result in improved COD performance in rugby players; 2) determine a suitable deceleration performance index that can be used in the field to assess deceleration ability. Methods: Seven male club level (age: 24.4 ± 7.4 years; height: 181 ± 7cm; mass: 87.5 ± 7.7kg; rugby experience: 10.9 ± 5.5 years) and five female regional level (age: 23 ± 5.7 years, mass: 71.3 ± 13.8kg, rugby experience: 6.8 ± 5.9 years) rugby players were recruited during a period of pre-season training. During one single session, the male group performed 10m speed, horizontal jump, vertical jump, bounce drop jump, 505 COD test, One Repetition Maximum (1RM) squat as part of pre-season testing. The female group completed the 505 COD test only. Participants then completed the six week training intervention which consisted of a 20-30 minute warm-up protocol twice per week. The protocol focused on deceleration drills and the execution of rapid deceleration efforts. All performance tests were repeated following the training intervention. A Paired T-Test, Effect Size (ES) and Hedges’ g ES calculations were used to analyse differences pre to post intervention. Smallest Worthwhile Change calculations were calculated to detect meaningful change pre-post intervention. Pearson Product Moment Correlation data using pooled pre and post intervention data determined relationships between performance tests and 505 COD performance. Results: Nine out of twelve participants improved or maintained performance in the 505 COD test (Group: n=12, 0.1 ± 2.8%, ES=0.09, Female: n=5, 0.06 ± 3.3%, ES=0.08, Male: n=7, 0.06 ± 2.4%, ES=0.10). There were significant improvements in jump performance, particularly bilateral vertical (ES=0.13), right leg (ES=0.37), and left leg (ES=0.83) vertical jumping pre to post intervention. There were very high correlations between relative strength (r=0.710) and 1RM squat (r=0.546) and 505 COD performance. High correlations between bilateral horizontal jump (r=0.543), and right leg horizontal jump (r=0.603), bilateral vertical jumps (r=0.606), right leg vertical jump (r=0.591) and left leg vertical jump (r=0.557) and 505 COD performance were found. A Deceleration Index has been proposed that provides accurate description of performance changes with regard to COD and momentum. Conclusion: Practicing COD movements and drills, specifically the deceleration component rapidly from high velocity running can improve COD performance and contributing sub qualities of this ability in rugby players. Practical application: Introducing deceleration drills as part of a warm-up for team trainings will provide enough stimulus to improve deceleration capacity and COD and agility performance. Drills should be progressed as players become more skilled, being able to decelerate from increasing speeds within shorter distances.