Advances in smart digital healthcare have driven innovations in soft and deformable biodevices, including sensors, prostheses, and surgical tools, which have significantly revolutionized precision and personalized medicine for applications in fitness tracking, health monitoring, human-machine interfaces, and disease forecasting. [1][2][3][4] There are various different implantable and wearable biodevices, which all must effectively capture physiological data and/or aid the ability of the body to carry out certain functions. Biodevices range from commercialized examples such as heartbeat sensors, hearing aids, and pacemakers, [5,6] to recently advanced wearable biosensors for body motion and biofluids analysis. [7][8][9] The need of transducing biological interactions into readable signals entails the necessity to develop soft and deformable bioelectronic devices that are wearable, ingestible, or implantable. In recent years, such a fast-growing interdisciplinary field of research has attracted extensive interest from device engineers and materials scientists. In contrast with traditional rigid electronics, soft and deformable biodevices can efficiently capture high-quality signals of patients unobtrusively due to their elastic and conformal characters. Rigid wearable devices can be uncomfortable to the wearer, and the high stiffness of implantable devices has been shown to be a major cause of foreign body reaction, causing inflammation and scarring. [10,11] It would be beneficial to use soft and stretchable biodevices instead, to reduce these negative effects. To make flexible electronic circuits in biodevices, one commonly used strategy is to embed intrinsically soft electrical conductors and interconnects in elastomeric materials.Metals that are in a liquid state at/near room temperature uniquely offer both metallic and fluidic properties, thus, providing the best combination of conductivity and deformability of any known materials and offer great potential as a way to create a range of soft, deformable biodevices. Gallium and its alloys such as eutectic gallium indium (EGaIn) and gallium indium tin (Galinstan) are the liquid metals focused on here. These liquid metals are low-toxicity alternatives to mercury, so are therefore relatively safe to use in biomedical devices and other biomedical applications. [12] For example, liquid metals have been used as a carrier for drug delivery, [13] and exploited for enhanced cancer therapy, [14] among other uses, as comprehensively reviewed by Yan et al. [15] Liquid metals have also been used to create an extensive range of flexible electronic components and devices. Examples include memristor-like devices, [16] strain sensors, [17] and antennas, [18] as extensively reviewed by Dickey. [19] See a recent review by Kim et al. [20] for a discussion of smart, stretchable electronics using a broader range of materials.This Review seeks to highlight exciting advances in the recent developments of gallium-based liquid metals within the context of wearable, implantable, and other...