Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications 1 , 2 , 3 , 4 , 5 , 6 , with stereolithography a particularly successful approach 4 , 7 , 8 , 9 . However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion 3 , 7 , 8 , 9 . Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures 4 , 5 , but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion 10 , 11 , 12 , 13 to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing.