Nanothermometry is the study of temperature at the submicron scale with a broad range of potential applications such as cellular studies or electronics. Molecular luminescent based nanothermometers offer a non-contact means to record these temperatures with high spatial resolution and thermal sensitivity. A luminescent based molecular thermometer comprised of visible-emitting Ga 3+ /Tb 3+ and Ga 3+ /Sm 3+ metallacrowns (MCs) achieved remarkable relative thermal sensitivity associated with very low temperature uncertainty of Sr=1.9 %K-1 and δT < 0.045 K, respectively, at 328 K, as an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs. They are so far the ratiometric molecular nanothermometers offering the highest level of sensitivity in the physiologically relevant temperature range. Nanothermometry is the analysis of the temperature of submicron systems. This domain of science offers potential applications in fields such as electronics, [1] microfluidics, [2] nanomedicine, [3] and cellular studies [4] where minimal spatial-sized thermometers are desired. Luminescence based thermometry correlates temperature with either emission intensity or emission lifetime changes of luminescent systems located in the studied environment. A low temperature uncertainty (δT) and a high relative thermal sensitivity of measurements (Sr) are desirable properties. Dual-centered ratiometric luminescent thermometry is a proven intensity-based method. In this technique, the intensity ratio (Δ) of two unique emitters' emission wavelengths provides a measurement of temperature with an internal calibration. [5] Molecular luminescence thermometry approaches what is perhaps the physical limit of attainable optical spatial resolution of temperature, restrained only by the diffraction limit of the optics used for detection. [6,7] It is difficult to generalize the parameters related to molecular thermometers, since while a system may excel in some respects it may be lacking in others. Molecular thermometers have been created using organic fluorescence dyes or transition metal complexes or clusters. [3,6,8-12] However, such systems have drawbacks since they usually exhibit limited photostability that depends on the intensity and duration of the light exposure, and can suffer from fluorescence intensity changes that are not related to variations of temperature but to modifications of pH, solvent viscosity or polarity, biological environment, or ionic strength. The wide emission bandwidths of organic reporters can complicate the interpretations of results in practical applications, and moreover, the luminescence lifetimes are most often used in the evaluation of temperature in such systems, with inherent drawbacks such as longer acquisition times and the requirement of rigorous post-processing data treatment as well as the inadequacy in mapping temperature gradients. [5] Lanthanide-based nanothermometers [13] may derive their functionality from the temperature-dependent luminescence of lanthanide cations (Ln 3+) in their compounds. Unique Ln 3+ photophysical properties originate from their core-like valence 4f electrons. These cations possess element-specific emission profiles with narrow bands the wavelengths of which are not affected by the experimental conditions. As most f-f transitions are forbidden, free Ln 3+ are very weakly absorbing resulting in low emission intensity. This limitation can be overcome through sensitization by an appropriate coordination environment. [14] While thermometers incorporating Ln 3+ are relatively common among metal-organic frameworks (MOFs) and other solid-state compounds