Transition metal complexes play a crucial role in antitumor therapy. Complexes of platinum, ruthenium as well as lanthanum and gallium have been investigated in preclinical as well as in clinical studies. The best known platinum (II) agents approved worldwide, Cisplatin or carbplatin, are used in nearly 50% of all cancer therapies. Due to their severe toxicity, their high activity is not always satisfactory and side effects are frequently encountered. Therefore, ongoing efforts word wide is aiming to identify novel metal-based substances with less toxicity and new mechanisms of action.