Clock references are indispensable circuit modules. The Internet of Things (IoT) devices, in particular, usually require a reference with high spectral purity for the phase-locked loop to generate the carrier signal and a low-power always-on reference to serve as the timer. This chapter discusses the design of two clock references operating at ultra-low supply voltage (<0.5 V) for energy-harvesting Internet-of-Things sensor nodes. The first is a sub-0.5 V 16/24 MHz crystal oscillator with a fast startup feature to accommodate the periodic duty-cycling scheme of the Internet-of-Things device. We prototyped the design in 65 nm CMOS (complementary metal-oxide semiconductor). The second is a 0.35 V 2.1 MHz fully integrated relaxation oscillator implemented in 28 nm CMOS. It features an asymmetric swing-boosted RC (resistor-capacitor) network and a dual-path comparator to surmount the challenges of sub-0.5 V operation while achieving temperature resilience. This chapter elaborates both designs in detail.