Ultra low power wireless sensor networks offer large opportunities for ubiquitous sensing and ambient intelligence, in applications such as industrial production, logistics, human health care and comfort, environmental and energy monitoring, etc. This invited talk discusses hardware solutions for such sensor networks when they are constrained by low available energy. A system solution for the network is presented, and a power/performance optimal architecture for the sensor nodes is proposed. Pulsed UWB communication provides low power as well as good locationing capabilities. Also the energy scavenging aspects for autonomous sensor nodes are discussed.