Understanding exercise intensity is essential for optimizing training outcomes and minimizing health risks. This chapter introduces key physiological and subjective parameters used to assess exercise intensity, including heart rate reserve (HRR), oxygen uptake reserve (VO2R), maximal oxygen consumption (VO2max), and ratings of perceived exertion (RPE). Standardized classifications from organizations such as the American College of Sports Medicine (ACSM) are presented, alongside practical methods like the Talk Test for field applications. Incremental exercise testing is highlighted for identifying physiological thresholds, including lactate and ventilatory thresholds, which serve as critical markers for personalized training. Additionally, recent advances in neuroimaging—including electroencephalography (EEG), near-infrared spectroscopy (NIRS), and functional magnetic resonance imaging (fMRI)—are reviewed to explore how different exercise intensities affect brain activity. Evidence suggests that even low to moderate-intensity exercise can positively influence cognitive function and cerebral blood flow. The integration of wearable technologies has further enabled real-time monitoring of both physiological and neurocognitive responses. Overall, this chapter underscores the importance of individualized, evidence-based approaches in exercise prescription and highlights emerging methods for linking exercise intensity with brain function.