Adolescent idiopathic scoliosis (AIS) is a common but complex three-dimensional (3D) deformity of the spine. When untreated, idiopathic scoliosis has a risk of progression. To prevent surgical treatment, patients are treated with a brace during their adolescent growth spurt with the aim of maintaining the curve below 45 degrees. Brace treatment is, however, not successful in every patient and there is room for further improvements. The general aim of this thesis was to explore the possibilities of using a biplanar low-dose X-ray device (EOS) as a tool for spine related measurements, and to expand the knowledge about factors associated with brace treatment success in AIS. In the first part of this thesis, two validation and reliability studies are described, reporting that spine length and pedicle size measurements on EOS radiographs are valid and reliable. The second part of this thesis investigated which factors are associated with increased initial in-brace correction, since lack of initial in-brace correction is associated with brace treatment failure. Strong evidence was found for increased curve flexibility as favourable predictive factors for initial in-brace correction. Also, a retrospective study is presented in the second part of this thesis, comparing two standardized protocols for radiological follow-up (in-brace versus out-of-brace radiographs), which shows a similar curve progression rate over time between the two groups. Lastly, the Dutch version of the Brace Questionnaire, measuring health-related quality of life, was studied and considered valid and reliable for the Dutch population group during brace treatment.