This is an image of down-conversion nanoparticles, a special class of optical materials that enable us to see them through human tissue! The key to their imaging power is this: they are fluorescent in in the near-infrared light spectrum, where biological tissue is almost transparent. With the right kind of laser and detector (that we have in our lab), we can image and visualize the movement of these nanoparticles throughout the body. These nanomaterials enable deep-tissue optical imaging, which is safer, cheaper and faster than conventional imaging technologies that are available today, leading to exciting applications in biology and engineering. We have used these nanoparticles to target and image ovarian tumors, and we are now using them to non-invasively visualize the movement of immune cells in cancer models.