This chapter presents a family of current-mode relaxation oscillators that can be designed either as a compensated digital clock source, or as an oscillator-based sensor whose frequency reports the temperature or supply voltage. One compensated timer implementation in 0.18 $$\upmu $$μm CMOS achieves a figure of merit of 120 pW/kHz, making it one of the most efficient relaxation oscillators reported to date. The oscillator design is then extended to produce a $$V_{DD}$$VDD-controlled oscillator and a temperature-controlled oscillator. Finally, we introduce a low-power hybrid oscillator sensor, which encodes measurements of both the supply voltage and temperature into the durations of its two alternating digital clock phases. The underlying dual-phase current-mode relaxation oscillator and the resulting sensor circuits are easy to implement, are area- and energy-efficient, and offer straightforward power and speed tradeoffs for a wide range of applications.