This book introduces the principle, main parameters, acquisition mode and operation procedure of Optical Quality Analysis System II (OQAS II) based on double-pass technology. OQAS II is increasingly used in the diagnosis and assessment of ocular diseases, especially for cataract. In this book, the clinical practice of OQAS II in the field of refractive cataract surgery is emphasized, including the preoperative/postoperative evaluation of visual quality, the timing of cataract surgery, the prediction of the visual quality after cataract surgery and doctor-patient communication. We hope the book provides a reference for the clinical application of the OQAS II in the field of cataract and helps to form the worldwide standard specification to a certain extent.