Learning Objectives: 1) State one unique advantage of PET measurement of tumor blood flow. 2) Detail two significant disadvantages of PET measurements of blood flow and blood volume. 3) Define a specific situation in which ultrasound measurements of tumor blood flow can be performed. 4) Distinguish between ultrasound's capabilities without contrast agents, with the usual blood pool agents and with targeted agents. Disclosure: (P.L.C. has a research agreement with General Electric.)