Photochemical processes driven by ultraviolet (UV) light are ubiquitous and span diverse applications such as photocatalysis, 1-3 photopolymerization, and biomaterial manipulation for cell culture or drug delivery. However, administering UV photons beyond the surface of a material (e.g., deep into a solution or inside biological tissue) is extremely challenging due to high absorption coefficients and scattering losses. One strategy to deliver high energy photons at depth is the application of an extremely high light intensity, but this approach wastes energy at the surface and may cause unwanted reactions that further degrade or damage the material. Upconversion (UC), the process of converting low energy photons into higher energy photons, has received significant attention to tackle light penetration issues because visible and infrared photons are generally more transmissive through materials compared to UV photons. Using UC, higher energy photons (e.g., UV photons) can be locally generated much deeper into a reaction matrix to activate light-driven reactions using low energy input photons. Among the many reported upconversion pathways and materials, triplet-triplet annihilation upconversion (TTA-UC) stands out: high UC efficiencies can be accessed using relatively low input powers, and a wide range of excitation and emission wavelengths have been demonstrated by tuning the system components. In addition to its potential for high efficiencies and materials tunability, TTA-UC imparts a further benefit inherent in its quadratic nature. Namely, upconversion intensity scales quadratically with input power below a threshold intensity, so precise spatial control of high energy photon generation can be achieved due to efficient upconversion occurring only near the focal point of focused input light. This property has been leveraged to demonstrate TTA-UC-enabled volumetric 3D printing using violet or blue upconverted light. The TTA-UC process is illustrated in Fig. . Sensitizers absorb low energy photons, generate triplets via spin-orbit coupling, and transfer the triplet states to annihilators through triplet energy transfer. Two annihilator triplet states can then undergo triplet-triplet annihilation to generate one high energy singlet excited state, which can radiatively decay and emit light with more energy than the incident photons. Annihilators for TTA-UC are usually small organic molecules like acenes, whereas sensitizers be a range of materials such as metallic complexes, thermally activated delayed fluorescence (TADF) molecules, and inorganic nanoparticles. For UC emission in the UV, a range of sensitizer and annihilator pairs for both blue-to-UV UC and green-to-UV UC demonstrate the flexibility of materials options, excitation wavelengths, and emission ranges available to accommodate distinct requirements for different light-driven applications. Yet despite this apparent versatility, sensitizers and annihilators used for UV light generation via TTA-UC are usually incompatible with aqueous environments. First, highly polar solvents tend to facilitate electron transfer reactions, which compete with the desired triplet energy transfers between sensitizers and annihilators. Additionally, high local concentrations of materials are required for efficient TTA-UC, but most organic TTA-UC molecules have limited solubility in water. While sensitizers and annihilators can be made water-soluble by appending polar functional groups (i.e., sulfate, carboxylate), this approach requires substantial synthetic efforts for each sensitizer and annihilator pair that could result in undesirable changes to their energetic properties and ultimate UC performance. Here, we employed nanoencapsulation as an alternate method to integrate UC systems into diverse matrices. After encapsulation, the chemical compatibility of the resultant nanomaterials with a system will dictate the efficiency of delivering upconverted light, thereby overcoming the limitations of individual molecules. We encapsulated UV-emitting UC materials using the block copolymer Pluronic F-127 to form self-assembled micelles in water. These micelles are comprised of poly(propylene oxide) cores surrounded by poly(ethylene oxide) coronas. UC micelles were fabricated by dissolving UC materials into a high-boiling point organic solvent (1, 2, 4-trichlorobenzene; TCB) that partitions into the hydrophobic core environment of the F-127 micelles. The nanoencapsulation method can generalize to different sensitizer and annihilator pairs providing the flexibility of materials choice and UV emission ranges. However, high solubility of the UC materials in TCB are critical to fabricate efficient UV-emitting UC micelles for their subsequent use to perform precise photochemistry at depth. .