Targeted therapies are increasingly being incorporated into standard treatment regimens for cancer. Tyrosine kinase inhibitors represent one class of targeted agents, with eight drugs receiving approval for the treatment of cancer in recent years. Although they hold the promise of being more effective and less toxic to normal cells than conventional cytotoxic chemotherapy, tyrosine kinase inhibitors are associated with different adverse effects, sometimes severe, as well as wide pharmacokinetic variability. Thus, a “therapeutic window” may exist for these agents.