Over the last 40 years, the clinical picture of chronic osteomyelitis without a microbiological cause has increasingly been described as chronic nonbacterial osteomyelitis/osteitis (CNO) in the sense of an entity. With the sharpening of the clinical picture, the disease designations became increasingly homogeneous. Consequently, an internationally agreed classification of CNO for childhood has recently been published, which is based on two studies on clinical diagnostic criteria. With regard to pathophysiology, findings have also shown that CNO can be categorised as an auto-inflammatory disease. However, it remains a diagnosis of exclusion. Quite a few differential diagnostic entities must be considered, which are almost indistinguishable from genuine CNO both clinically and morphologically. For the latter, a genetic or metabolic cause is still largely unknown, although various activations of inflammatory pathways have been described. In the meantime, the basis has been laid for conducting controlled studies in which, in addition to the classification, parameters for disease activity and suggestions for inactive disease and remission have been published.This review describes the historical development towards a now more clearly defined disease. It discusses diagnostic and therapeutic algorithms using the example of an adolescent patient with spinal and extremity manifestations.