For many decades, EPR has been used as a tool to study paramagnetic metalloproteins. Recent developments in multi-frequency pulsed EPR and ENDOR have significantly increased the amount of information that, in principle, can be obtained from such an EPR study, but at the same time, modern EPR spectroscopists are facing the difficult task of choosing the optimal techniques to tackle a specific problem. In this chapter, a number of strategies and their possible problems are outlined using different examples of recent multi-frequency EPR/ENDOR studies of metalloproteins and related model systems.