This dissertation presents circuits for wireless sensing, divided into two main parts. The first part focuses on receivers designed primarily for the mmwave range using sub-harmonic down-conversion to relax the phase noise and power consumption requirements of generating a local oscillator (LO) signal. An LTI analysis method for passive mixer-first receivers is introduced to simplify analyzing LC networks at the mixer input. A 4-phase sub-harmonic passive mixer-first receiver with improved reception at the third harmonic of the LO frequency is presented, using a class F$^{-1}$ LO buffer to shape the control the duty-cycle of the mixer switches’ conduction waveform and a shunt LC network at the front-end. Fabricated in INTEL 16 technology, the receiver demonstrates a noise figure of 9 dB, an out-of-band B1dB better than -18.63 dBm, and an out-of-band IIP3 better than 7.15 dBm, while consuming only 14.4 mW at around 60 GHz. The 4-phase design evolves into an 8-phase design, enabling reception at either the fundamental frequency or the third harmonic through output combination. An 8-phase design is also introduced for FMCW radar applications, incorporating an on-chip injection-locked class F$^{-1}$ oscillator. The second part addresses wireless micro-scale sensing systems for neural recording and pH measurement. The Micro-scale Optoelectronic Tetherless Electrode (MOTE) is the first wireless neural measurement system with a sub-nanoliter volume. It features on-chip integration of CMOS circuits and a microscale LED, allowing chronic in vivo neural activity recording in awake mice for over a year. The MOTE overcomes previous challenges in neural implants, including relative motion between the electrodes and the tissue and excessive volume displacement by implanted electronics. Building on the MOTE platform, a pH-measuring autonomous microsystem named the Redox-Enabled Microscale Opto-Electronically Transduced Electrode (ReMOTE) is introduced. The Re- MOTE contains two electrodes made of different materials, where the local pH influences the voltage between them. It is a tetherless, all-optical microsystem designed in a 180 nm CMOS SOI process, occupying just 417 $\mu$m $\times$ 48 $\mu$m and consuming 2 $\mu$W with a pH sensitivity of 42.53 mV/pH.