Section I Introduction: Types and Material Properties 1 Aligner Treatment: An Overview 2 Material Properties of Aligners Section II Clinical Management 3 Early Treatment in Preteens and Teenagers Using Aligners 4 Teen Treatments with Aligners 5 Routine Mechanics and Aligners (Extraction/Nonextraction) 6 Precautions with Aligner Treatment 7 Clear Plastic Appliances as Retainers 8 Digital Workflow in Aligner Therapy Section III Evidence 9 Outcome Assessment and Evidence on the Clinical Performance of Orthodontic Aligners 10 Forces and Moments Generated by Aligner-Type Appliances for Orthodontic Tooth Movement 11 Aligners and the Oral Microbiome 12 Intraoral Aging and Changes on Aligner Mechanical Properties 13 Color Changes of Aligners and Thermoplastic Retainers during Intraoral Service 14 Biological Properties of Aligners 15 Aligner Treatment from the Patient Perspective