Thoracolumbar malunion is the result of loss of correction, insufficient correction or even no correction (both in the frontal and sagittal planes) of a thoracolumbar fracture. The main causes are incorrect assessment of the fracture's complexity (burst fracture), its potential progression to kyphosis and associated disc or ligament damage. It can also be the result of a poorly conducted initial treatment. The types of malunion have changed over the years because of the introduction of vertebroplasty and kyphoplasty. The malunion can be well tolerated if there is only a moderate deformity. However, the functional and pain-related limitations can be severe with large deformities. Functional limitation is mainly related to sagittal imbalance, but also to sequelae associated with the injury in various ways (non-union, disc degeneration, spinal cord compression, syringomyelia, etc.). The deformity and its consequences are evaluated globally using full-body standing radiographs (EOS), CT scan and MRI. Comparison of MRI images taken in a lying position to weight bearing views or even dynamic ones is an additional means to evaluate whether the lesions are reducible. Differences in spine morphology and compensatory mechanisms to combat the sagittal imbalance induced by the deformity must also be analyzed. These provide more complete information about the consequences of the malunion and help to establish the best corrective strategy. These compensatory mechanisms consist of accentuation of lumbar lordosis along with reduction of thoracic kyphosis. As a last resort, the pelvis and femur contribute to this compensation when there is a large deformity or a stiff spine due to preexisting osteoarthritis. Treatment strategies are fairly well standardized. When the deformity is reducible, a two-stage surgery is indicated. When the deformity is not reducible, posterior transpedicular closed wedge osteotomy is the gold standard. Nevertheless, the best way to treat thoracolumbar malunion is to prevent it.