RFID`s and wireless sensors are being used in an increased number of applications including identification, tracking, monitoring, logistics and security. As a result, there is a growing need for highly efficient, environmental friendly, and energy autonomous nodes. Additionally, low cost, compact and conformal nodes are being sought. The presentation will review commonly employed energy harvesting technologies such as solar, electromagnetic, thermal and vibration, and provide performance limits and application scenarios as well as challenges associated with materials and integration. Finally, design efforts related to a flexible, hybrid solar/electromagnetic energy harvester will be presented.