Resistance training muscle adaptations may be enhanced under increased environmental temperatures (>30 °C). Training in the heat has been widely used to enhance performance adaptations in a range of populations from untrained to elite-athletes. Environmental heat stress is used to provide an additional training stimulus to the individual to potentiate greater physiological adaptations and subsequent athletic performance. Chapter One of this thesis reviews the current literature on different methods of thermal stress and how it could be implemented for preparation of rugby union athletes. Chapter Two of this thesis includes an original investigation whereby 18 elite, semi-professional rugby union athletes (age [mean ± SD], 22.2 ± 3.5 years; body mass 99.6 ± 11.5 kg; height 187.6 ± 6.4cm) performed a three-week resistance training intervention (12 sessions) where they were randomly allocated into two groups: HEAT (n = 8) with all lower-body resistance training sessions performed in environmental conditions maintained at 35 °C and 37% relative humidity (RH), or CON (n = 10) where the same training was performed in temperate conditions (21 °C and 45% RH). Pre and post-training intervention tests included measures of strength, power, endurance, speed, and body mass. When comparing groups, small effect sizes were found in favour of HEAT for the back squat (d = 0.26) and bench press (d = 0.23). All other measures were associated with trivial or unclear effects. A significant group x time interaction for body mass, associated with a trivial effect size (d = 0.19, HEAT +1.5 kg; CON -0.8 kg). Resistance training in the heat may lead to small improvements in lower and upper body strength compared to the identical training programme performed in temperate conditions. Utilization of heat stress during specific strength training sessions may prove to be a beneficial strategy to increase muscle adaptions for rugby union athletes.