This contribution discusses different aspects related to EM- and piezo-scavengers from ambient energy that are very promising as power suppliers of the latest evolution of ultra-low power electronic systems. First challenges in designing circuits and systems for microwave ambient energy harvesting are discussed. A design flow, including the RF/Microwave receivers and the base-band conversion blocks is presented. Power densities as low as few μW/cm2 are targeted as realistic sources to be scavenged by a highly efficient multi-band resonant rectenna. Predicted and measured results in stationary regime are presented and the measured converted energy is also assessed. Second, a conventional piezoelectric scavenger is tested as a power source of a nomadic RFID-reader adopted for novel localization system approach. The latter is intended as an example of multidisciplinary (RFID-NFC, ultra-low power systems, short range wireless power transfer, organic technologies and so on), fully autonomous (energy scavenger on board, battery-less), wirelessly empowered (multitude of passive tags is energized by autonomous reader), evolutionary electronic system toward a “greener world”.