The emerging field of internet of things promises mankind an enhanced life quality, produc-tivity and security. One critical technology enabler is ubiquitous and unobtrusive wireless connectivity activated by ambient events and operated with little human intervention for con-figuration and maintenance. Commercial off-the-shelf radio devices cannot achieve the desired performance, reliability and ultra-low power consumption around 100µW at the same time. In this work, research is carried out on the design and implementation of an ultra-low-power radio for generic wireless event-driven applications including healthcare, information and enter-tainment, industrial and home automation, as well as environment monitoring. To fulfill the stringent power budget, the envelope detection and the direct-modulation are the architectures of choice for receiver and transmitter front-ends, respectively. However, such radios suffer from poor sensitivity and frequency selectivity, and thus are unable to op-erate reliably across the desired link distance or in the presence of interference. This work investigated the root causes of insufficient sensitivity and selectivity in envelope detection receivers, and proposed design guidelines to optimize their performance. Furthermore, two novel envelope detection schemes have been proposed. The synchronized-switching technique improves the sensitivity by suppressing DC offset and 1/f noise in the receiver, while the 2-tone signaling technique enables in-band interference rejection which was not possible in prior arts. Prototype circuits have been built to verify the proposed techniques. On a 90nm CMOS technology, a transmitter and a receiver front-end are designed to benchmark the performance of 2-tone envelope detection in practice. The digital-IF, direct-modulation transmitter carries out the 2-tone IF-PSK modulation with -6dBm output power while consuming 893µW. The 2-tone envelope detection receiver realized up to 282 times improvement in interference re-jection while dissipating between 63.5µW and 121µW. A link budget of over 80dB is realized by this transceiver pair, which translates to a link span up to 30 meters in indoor environments and 100 meters outdoors. By following a systematic approach, devising innovative architectures, and optimizing circuit performance, this work has confirmed the feasibility of ultra-low-power, autonomous and robust event-driven radios in low-cost and commercially available CMOS technologies.