The pharmacotherapy of tumor pain has two main aims: to deliver an adequate basic analgesia using long-term retarded opioid medication and an effective treatment of tumor breakthrough pain using rapidly effective non-retarded opioids. Breakthrough pain is characterized by a sudden onset and rapid increase in the pain level and should be treated with correspondingly rapidly effective opioids. The pharmacological characteristics of previously available and routinely prescribed non-retarded opioids do not always correspond in oral galenics to the demands resulting from the definition of tumor breakthrough pain. As alternatives to these substances five different rapidly effective fentanyl preparations are now available for transmucosal administration.