During the past decades, diagnostic imaging in the field of oncology has been mainly concerned with morphology. Molecular imaging is emerging paradigms that may enable clinicians localize tumors; visualize the expression and activity of specific molecules responsible for or related to the disease. Although developments of numerous promising strategies are underway, clinical applications of such techniques are quite limited to date. We review the current status of the field mainly referring to clinically available positron emission tomography and magnetic resonance imaging.