Non-unions are a relevant medical and socio-economic problem. Hyper-, oligo- and atrophic non-unions as well as septic and aseptic non-unions are differentiated. Correct classification is essential for the selected therapy. The "diamond concept" describes five pillars, on which bone healing is based and that have to be considered in the treatment of non-unions: osteogenic cells (mesenchymal stem cells), osteoinduction (growth factors), osteoconduction (scaffolds), mechanical stability, and vascularization. Factors that predispose to non-union also influence fracture healing. The gold standard of therapy are still resection of the non-union, decortication and autologous bone grafting. No advantage could be proven for any of the numerous procedures in monotherapy. But the combination of various procedures - polytherapy - seems to be promising. The aim is to optimize these concepts.