In children with low back pain (LBP), a specific cause is often identified. LBP has a relatively high prevalence during school years. However, only a minority of the children suffering from LBP seek medical attention. Protracted back pain in childhood is a serious condition that should be thoroughly investigated. This article is a systematic review of the intrinsic causes of LBP. Imaging modalities are discussed, with emphasis on magnetic resonance imaging. We have divided the intrinsic causes of LBP into four main groups: mechanical, developmental, infectious/inflammatory, and neoplastic. Disk protrusion is prevalent in young athletes. Spondylolysis and spondylolisthesis are the most common causes of chronic LBP in children. Thoracic or thoracolumbar Scheuermann disease causes kyphosis while a lumbar localization is more painful. Childhood diskitis is associated with fever and leukocytosis. Spinal inflammatory arthritides in children include juvenile rheumatoid arthritis, the juvenile spondyloarthropathies, and SAPHO syndrome, where spine as well as sacroiliac joint changes may be seen. Cysts, tumors, tumor-like lesions, and metastases are infrequent causes of back pain in children. Several of these conditions are described and illustrated in this review of LBP in children and adolescents.