The aim of this systematic review is to search any relevant literature on any changes in aligner thickness after thermoforming process. This Review was registered at PROSPERO no. CRD42022371213. A systematic search was performed through five databases and hand-searching of the reference lists of the included studies. Reporting is based on the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines. Risk of bias assessment was based on the Cochrane Risk of Bias Tool. A total of 245 eligible studies were identified but only five studies were included for quality assessments. Aligners in the studies were made from three different materials (PETG, PU, and SmartTrack). The thickness values were compared by arch, tooth and location points. The analysis between upper and lower aligners fabricated from SmartTrack showed a significant difference in the molar lingual region when comparing upper (0.631 mm) and lower aligners (0.563 mm). Significant differences by tooth were also detected between first molar–incisors and first molar–canine. However, no significant difference was found between incisors and canines. All three different materials reported higher thickness values on cusp tips compared to gingival region. Aligner thickness was only 44% - 95% with respect to the original material thickness. Inhomogeneity of thickness was detected to be higher at the posterior region, particularly on molar cusp tips, and lower toward the anterior region. © (2023), (University of Dicle). All Rights Reserved.