Heat stress in dairy cattle is a major and growing welfare challenge as climate change extends and intensifies hot periods globally. Dairy cows are most affected due to the metabolic heat generated by high milk production. The Temperature Humidity Index (THI) above 68 triggers measurable welfare and production effects: reduced feed intake, increased water intake, elevated respiration rate, reduced activity, and impaired reproduction. Research demonstrates cows under heat stress show altered lying behaviour, spending less time lying and more time standing near water and shade. Cooling systems - shade, fans, sprinklers, and misters - are highly effective: evaporative cooling reduces rectal temperature by 0.5-1.5°C and restores production within hours. Economic analysis shows cooling investment pays back within 1-2 seasons through maintained milk production and reproductive performance. Genetic selection for heat tolerance is an emerging long-term adaptation strategy. Climate-smart farm design incorporating cooling infrastructure from construction is becoming standard in warm-climate dairy regions.