This paper describes two wireless power transfer systems. The Wireless Identification and Sensing Platform (WISP) is a platform for sensing and computation that is powered and read by a commercial off-the-shelf UHF (915MHz) RFID reader. WISPs are small sensor devices that consume on the order of 2uW to 2mW, and can be operated at distances of up to several meters from the reader. The second system harvests VHF or UHF energy from TV towers, with power available depending on range and broadcast transmit power. We report on an experiment in which 60uW is harvested at a range of about 4km.