Background: Maximal oxygen uptake (VO₂max) is widely recognized as the gold-standard indicator of cardiorespiratory fitness and an essential determinant of endurance performance, clinical prognosis, and physiological adaptation to training. VO₂max can be quantified directly through laboratory-based gas exchange analysis or indirectly estimated using field-based performance tests. Despite widespread application of both approaches, uncertainty persists regarding their comparative accuracy, validity, and reliability in athletic populations. Objective: To systematically compare laboratory-based (direct) and field-based (indirect) methods of VO₂max assessment with respect to measurement accuracy, criterion validity, and test–retest reliability in athletes. Methods: A systematic review was conducted in accordance with PRISMA guidelines. Peer-reviewed studies comparing directly measured VO₂max obtained via graded exercise testing and open-circuit spirometry with estimates derived from field-based protocols (e.g., Bruce protocol adaptations, Cooper 12-minute run, Yo-Yo Intermittent Recovery Test, and multistage shuttle run tests) were included. Methodological quality, validity coefficients, reliability indices, and estimation errors were extracted and synthesized. Results: Laboratory-based assessments consistently demonstrated superior accuracy and served as the criterion reference standard. Direct measurement showed minimal technical error and high reproducibility under standardized conditions. Field-based tests exhibited moderate-to-high criterion-related validity (typically r = 0.70 – 0.90) and good-to-excellent reliability when protocols were standardized. However, systematic over- or under-estimation and prediction error were frequently reported, particularly when regression equations were applied beyond their validated populations. Conclusion: Direct laboratory measurement remains the most accurate and valid method for assessing VO₂max in athletes. Nevertheless, field-based tests provide reliable, cost-effective, and ecologically valid alternatives for large-scale screening and sport-specific monitoring when laboratory testing is impractical. Selection of assessment method should therefore consider the required level of precision, available resources, and contextual application.