The highest accessible formal oxidation states of the d-block elements are scrutinized, both with respect to the available experimental evidence and quantum-chemical predictions. The focus is on fluoride, oxide, and oxyfluoride systems. The field has evolved significantly over the past 15 years due to the availability of quantitative computational predictions of thermochemical stabilities, and of spectroscopical parameters of a number of key molecules. The demands on the computational methodology used, as well as the experimental boundary conditions needed are reviewed, and reasons for the observed trends in oxidation states throughout the 3d, 4d, 5d, and 6d series are discussed.