In the last decade, there has been a significant focus on examining the effects of Ramadan fasting on Muslim athletes. Ramadan, one of the five pillars of Islam, mandates that nondisabled Muslims refrain from eating, drinking, engaging in sexual activity, and smoking from sunrise to sunset for 29–30 days each year. Restricting energy intake to nighttime, along with additional social and religious activities, can disrupt regular sleep and normal eating patterns for those who fast, which may negatively affect athletic performance. Scientific studies have shown conflicting findings regarding the impact of Ramadan observance on athletic performance. Systematic reviews with meta-analyses have found that Ramadan fasting has a negative effect on mean and peak power during afternoon anaerobic Wingate cycle exercises, as well as on morning sprint running performance. However, no associations have been observed with strength, jump height, fatigue index (FI), or aerobic performance. Despite the lack of adjustments to the sports calendar to accommodate religious requirements, Ramadan fasting presents a challenge for Muslim athletes who are required to compete or prepare for important events while fasting. They are unable to consume nutrients and fluids for recovery during daylight competitions or training sessions, which can negatively affect their hydration and nutritional status. Additionally, Muslim athletes observing Ramadan may face a competitive disadvantage depending on their sport, the time of day, and the environmental conditions during physical activities. This chapter examines the impact of Ramadan fasting on performance during exercise testing and explore the potential mechanisms behind the performance decline noted in some studies during this month.