Multi-photon lithography (MPL) remains among the handful of practical techniques that can be used to fabricate truly three-dimensional (3D) micro- and nanometer-scale structures with few processing steps. Although photopolymers remain the primary material system for MPL, others have been developed for creating functional structures in chalcogenide glasses and polymer-composites. Post-exposure processes have been developed for transforming a structure created by MPL into another material, such as a metal, semiconductor, or oxide glass. MPL has been used to create a wide range of functional nanophotonic devices. The full potential of MPL is apparent in its use to create spatially-variant lattices (SVLs). SVLs are a new class of nanophotonic device that is engineered to direct and control the flow of light in 3D. The devices are based on unit cells that control the propagation of light through the selfcollimation effect. These unit cells are spatially varied in orientation throughout an SVL so that light flows along a prescribed path within the device. The geometry and patterns of the unit cells within the lattice can also be varied to control other key properties, such as phase and polarization. SVLs and their fabrication by MPL opens a new route to 3D integrated photonics, and a myriad of other applications.