In this paper, we summarize research activities and technology progress in the field of current CMOS-MEMS resonators and oscillators for the portable sensor node and timing applications. By employing CMOS-based fabrication technologies, we can monolithically integrate MEMS devices and their associated application-specific integrated circuits (ASICs), thereby enhancing their overall performance as compared with their stand-alone counterparts. Owing to the diversity of post-CMOS processing techniques, in this review, we mainly focus on the devices achieved by so-called foundry-orientated CMOS-MEMS platforms. On the basis of how the mechanical structure is achieved, in the paper, we focus on two major strategies: (i) additive back-end-of-line (BEOL) compatible layers and (ii) machining of standard CMOS layers for device fabrication. Given their superior CMOS integration capability, the circuitry design concepts, testing results, and potential merits of state-of-the-art CMOS-MEMS oscillators are also presented.