Body composition and nutrition play a vital role in both health and sporting performance. Optimising energy and macronutrient requirements for a team, position, and/or individual, can enhance an athlete’s preparation, performance, and recovery from training and competition. The requirements of professional Rugby Union (RU) are unique given players can be very large, strong, and powerful, or much smaller, faster, and elusive. Regardless of their size or respective playing position, players must be able to withstand numerous high-impact collisions, yet, be fit enough to play 40-80 minutes at high-intensity week after week. Therefore, positional and/or individualised body composition and fuelling requirements are required. Through a series of seven studies, this PhD thesis aimed to enhance the understanding of body composition and nutritional requirements for professional RU players. Study One assessed the physical and fitness characteristics of elite male professional RU players from New Zealand (NZ) by primary playing position. Study Two examined the physical characteristics of elite female RU union players from NZ between and within different playing positions. Study Three assessed the body composition and anthropometrics by specific playing position and playing level among professional male and female RU players from NZ. A newly developed skinfold-based prediction equation was created to estimate body fat percentage (BF%) among male and female forwards and backs. Study Four explored the competition nutrition practices of elite male professional RU players from NZ over seven-days which included game-day (GD). Study Five examined the dietary intakes of elite male professional RU players from NZ in catered and non-catered environments, both during competition weeks which included GD. Study Six measured the resting metabolic rate (RMR) of professional male RU players from NZ and developed new RU-specific prediction equations. Finally, Study Seven evaluated the in-season energy demands of elite male professional RU players from NZ, providing specific training session and game-play energy demands by position. In Study One, at the end of pre-season, body composition was assessed among elite male professional RU players (n = 39) using sum of eight-site skinfolds (Sum8SF) and Dual-energy X-ray Absorptiometry (DXA), alongside fitness (strength, power, speed, and aerobic fitness) measures. Forwards possessed significantly greater body composition measures compared to backs. Both forwards and backs within this study were taller, heavier, and leaner than players from previous studies in the northern hemisphere. Among forwards, Sum8SF demonstrated very large correlations with relative power and speed measures, while backs demonstrated large correlations between Sum8SF and aerobic fitness. Fat mass and BF% demonstrated very large correlations with speed and aerobic fitness measures among forwards. Skinfolds, fat mass, and BF% relate strongly with key fitness characteristics required for elite male professional RU performance. In Study Two, the body composition of elite female RU players (n = 30) was assessed using Sum8SF and DXA during the in-season period. Forwards were significantly taller, heavier, and possessed greater Sum8SF than backs. Forwards also possessed significantly greater total and regional body composition measures compared to backs. Healthy bone mineral density measures were observed among both forwards and backs. Significant differences were also present among forward positional groups. Study Three extended on from Studies One and Two by examining the body composition (using DXA) and anthropometric (using Sum8SF) differences between specific playing positions and playing levels among professional male (n = 104) and female (n = 75) RU players. Elite players demonstrated significantly higher levels of lean mass than sub-elite players. Relationships between BF% and Sum8SF were large among female backs, and very large among male and female forwards and male backs. A new BF% prediction equation was developed using Sum8SF among male forward and back positions, and female forward and back positions. In Study Four, elite male professional RU players (n = 34) from NZ completed dietary intakes in-season using the Snap-N-Send method during a seven-day competition week which included GD. Forwards reported significantly higher absolute energy intakes, but no significant differences were observed for mean seven-day relative carbohydrate, protein, and fat intakes. Both forwards and backs reported their highest energy and carbohydrate intakes for the week on GD, with ~62% of total kcal being consumed prior to the start of the game. Players fell short of daily sports nutrition guidelines for carbohydrate and appeared to “eat to intensity” by increasing or decreasing energy and carbohydrate intake based on the training load. Study Five, extended on from Study Four by examining the dietary intakes of elite male professional RU players (n = 12) in catered and non-catered environments during two seven-day competition weeks which both included GD. No significant differences were observed for relative carbohydrate intake between catered and non-catered environments among forwards and backs. Whereas, mean seven-day absolute energy, relative protein, and relative fat intakes were significantly higher in the catered compared to non-catered environment among forwards. Meanwhile, backs presented non-significantly higher energy and macronutrient intakes within a catered compared to a non-catered environment. In Study Six, the RMR of male professional RU players (n = 108) were examined using indirect calorimetry, which were then compared to nine commonly used prediction equations. Compared to measured RMR for the group, seven of the nine commonly used prediction equations significantly under-estimated RMR, meanwhile one equation significantly over-estimated RMR. This led to the development and validation of two new RU-specific prediction equations which determined that the strongest predictors for this group were fat-free mass (FFM) alone, followed by body mass (BM) alone. Lastly, Study Seven examined the game-play, daily, and session specific exercise energy expenditure alongside energy intake during a competition week among elite male professional RU players (n = 24). Each player’s FFM was measured using DXA, which then allowed energy availability to be calculated. Changes in body mass and Sum8SF were then measured four-weeks apart to determine changes based on energy availability. This study also examined RMR on a rest day (three-days prior to GD [GD-3]) during the competition week and the morning after the game (GD+1). Significant differences between days, training sessions, game-play, and RMR between days were observed. Significant differences between energy availability groups and changes in body mass and Sum8SF were also present. In summary, the series of studies in this thesis provide valuable insight into the body composition and nutrition requirements of professional RU players. Practical strategies are provided to help practitioners estimate BF% if laboratory-based methods are not attainable, which can then inform FFM to be used in the newly developed FFM-based RMR prediction equation. However, a BM-based RMR prediction equation was also developed in case Sum8SF and FFM measures are not attainable. Finally, when the kcal·kg·min-1 factors are used to determine game-play and/or specific training session exercise energy expenditure, energy requirements can be determined specifically for professional RU players to optimise fuelling and/or body composition change strategies.