Introduction: Clear aligners are an aesthetic alternative to fixed appliances, yet their predictability in arch expansion is still debated. This systematic review evaluates the efficiency of aligners in achieving planned expansion and identifies factors influencing outcomes. Materials and Methods: A systematic review was conducted following the PRISMA guidelines. Electronic searches were performed in PubMed, Cochrane Library, and ScienceDirect using a PICOS-based strategy. Methodological quality and risk of bias were assessed using standardized tools (AMSTAR 2, NHLBI/NIH). Results: Nine studies were included in the final analysis, consisting of two systematic reviews and seven cohort studies. The review found that arch expansion predictability is highest in the premolar region (reaching up to 93.53%) but decreases progressively toward the posterior segments, with the lowest accuracy observed at the first molar level (approximately 55-68%). Expansion is primarily achieved through coronal tipping rather than true bodily translation. Conclusion: Aligners effectively manage mild crowding but often result in uncontrolled tipping. To improve predictability, clinicians should utilize overcorrection strategies and pre-program negative torque. Higher-quality randomized clinical trials are necessary to establish standardized protocols.