The development, operation and applications of a novel continuous chemical reactor system are described. The system, known as a Multiple Stream Mixer/Reactor (or MMR), produces nanoparticles by direct precipitation from two or more reactant solution streams in an extreme energy density, ultraturbulent, collision reaction region. Sensors and a control system are employed to assure constant mixing conditions and desired stoichiometry in the reaction region. The interaction chamber is designed to allow macro-, meso- and micromixing during processing. This allows control of product purity, yield, size, size distribution, and phase purity. Typical nanoparticle diameters in the one to ten nanometer range are often achievable, with tight size distribution. The MMR can be scaled from development quantities to tons/hour for production applications.