 The term laser likely conjures an image of a rectangular box plugged into a wall, or maybe of a glowing red dot on a screen. But in this study, researchers created what is effectively a miniature laser in a living biological cell. The microscale laser offers new opportunities for optically labelling cells and monitoring their internal environment. Luminescent probes such as quantum dots, bioluminescent molecules and plasmonic nanoparticles have allowed for great advances in cell imaging. Nevertheless, the degree to which these probes can target unique cells, like an individual cancer cell, is limited by the fact that they emit over a broad range of wavelengths in the visible region. Indeed, it becomes difficult to distinguish one emission signal from another when they overlap. The ability to induce laser emission in cells thus offers a means to vastly improve the performance of traditional imaging techniques due to the narrow emission line widths of lasers. To make any lasing action possible within a cell, however, the medium that supports laser emission and the optical cavity that confines that emission must be sufficiently biocompatible, small and efficient to operate within a cellular environment. In this study, the authors formed intracellular lasers using oil droplets, natural lipids and polystyrene beads as optical cavities measuring less than the width of a human hair. The researchers first injected oil droplets mixed with dye molecules into cells and observed their emission characteristics. By monitoring changes in laser emission with the deformation of the oily optical cavities due to cell mechanical stress, the researchers could detect fluctuations in force 10 times smaller than those registered by direct image-based analysis. The authors then examined the possibility of creating a truly biocompatible microlaser system by probing cells that naturally contain a lipid droplet, namely adipocytes or fat cells. Not only was lasing action determined to be feasible in extracted adipocytes, it was also sustained in adipocytes lying beneath the surface of pigskin. Finally, the authors showed that the medium-sustaining lasing action within cells could be manipulated. Specifically, lasing could be induced within polystyrene beads containing fluorescent molecules in the fluid that surrounds the different parts of cells and on the surface of fluorescent beads embedded in cells. For the polystyrene beads in particular, the authors demonstrated that different beads give rise to different laser spectra. This means that thousands of beads of different sizes could yield unique emission signals. And if each signal were associated with an individual cell, thousands of cells could be uniquely monitored. The authors offer several avenues for expanding the application of intracellular micro lasers, including the use of biodegradable polymers as the cavity material to improve compatibility with bodily tissues and the incorporation of semiconductor materials to push the cavity size to less than one micron, which would facilitate the injection of micro laser systems into various types of cells.