Osteosarcoma is a severe disease with low incidence but high mortality that occurs in young adults between 15 and 25 years of age. In order to improve the (still) low percentage of survival in patients, researchers are working, on one side, on imaging techniques and, on the other side, on new potential treatment approaches. This book is a PhD thesis that deals with both topics. Concerning the imaging, it is shown how PET imaging may be essential together with the routinely used diagnostic techniques (x-ray and biopsy) for the characterization of the phenotype of each individual osteosarcoma. In regard to the treatment, the efficacy of photodynamic therapy is evaluated for the treatment of osteosarcoma. In vitro the therapy is definitively efficient while in vivo more studies need to be performed for optimizazion of the settings. Both studies were so far conducted using pre-clinical animal models of the disease. If both applications reaches the clinics, this could represent an enormous step forward for the achievemt of a better survival of osteosarcoma patients.