The capability to customize the structure or composition of an optical element gives designers access to previously unrealizable configurations that show promise for reducing costs, enhancing functionality, as well as improving the size, weight, and power of optical systems. Techniques for three-dimensional (3d) printing of glass have opened the door to novel glass optics with both unconventional structures and tailored composition. An overview of the state-of-the art in glass 3d printing will be presented. Particular emphasis will be placed on the direct ink writing (DIW) technique, in which specially formulated silica pastes are extruded through a nozzle and deposited in the geometry of interest, forming low density green bodies. The green bodies are then converted to full density, optically homogeneous glass by a series of heat treatments. The 3d printed silica-based glass components have material and optical properties that rival conventionally prepared optical grade fused silica. In addition, glass optics that contain tailored gradients in composition, such as gradient index lenses, have been achieved by DIW by blending separate inks inline at the print nozzle and directly depositing the desired composition profile before forming the glass. Strategies are also being developed to reduce time to development of new materials and structures.