The macromolecules that make up our cells are in a constant state of activity, with their structures changing as they carry out their functions. Until recently, techniques capable of capturing high resolution structures of reacting biomolecules were limited to the relatively few reactions that are either light-activated or very slow. Mix-and-Inject Serial Crystallography (MISC) at X-ray Free Electron Lasers is an emerging technique that captures the structures of ligand-activated biomolecular reactions occurring on the millisecond timescale with atomic detail. This thesis describes the microfluidic devices and associated technology that were used for the first successful millisecond MISC experiment, opening up entirely new possibilities for structural biology. Further advances to the microfluidic technology and development of complementary methods to allow MISC to become reliable and routine are also discussed. The microfluidic devices described in this thesis use coaxial flow focusing to rapidly combine protein crystals and ligands. After mixing, a Gas Dynamic Virtual Nozzle propels a free-jet of the mixed sample into the X-ray beam. This technology has been employed at 10 X-ray Free Electron Laser Experiments to study a variety of biomolecular systems on timescales from 5 ms to 2 s. Further advances to the mixing injector technology make Mix-and-Inject experiments easier and more efficient are also described. This technology, which takes the form of a screw-together fluidic holder, allows rapid, modular fabrication of mixing injectors and requires less time, training and specialized equipment. This newly designed system allows for more flexibility when configuring mixing injectors for any particular experiment. Finally, this thesis details a complementary methodology employing Rapid Freeze Quenching and Electron Paramagnetic Resonance (EPR) to characterize Mix-and-Inject parameters ahead of beamtime. This method allows reactions in EPR-active microcrystals to be monitored in the lab using the same mixing injectors, flow rates and crystallization conditions as a Mix-and-Inject experiment. This new method is demonstrated by tracking the reaction between azide and myoglobin, for both solubilized and crystalline protein.