This chapter reviews the origins of continuous hydrothermal/solvothermal synthesis and their respective technical and engineering links back to batch synthesis. The mechanisms of formation via nucleation and growth stages are discussed and how these stages can be controlled through rapid mixing of a preheated fluid with a cold flow carrying an aqueous metal salt. The difficulties in moving from batch to continuous synthesis are also discussed, as well as how many researchers across the world have attempted to solve these issues using modelling and empirical measurements of the continuous mixing process. The modelling approaches include computational fluid dynamics and pseudo-fluid mixing, which uses surrogate fluids to represent the two fluids inside the high-temperature high-pressure vessel. Empirical measurements have either been via trial and error experiments using different reactor designs or with neutron beam experiments able to actually visualise the mixing in situ. These approaches have all contributed to the 'rules' around the design of continuous reactors for continuous flow synthesis. These 'rules' are discussed in detail. The road to scale up is also discussed in the chapter whereby bench scale developments have been commercialised via pilot scale systems on to full scale systems at 10-1000 tons per year. The benefits of the continuous process are also discussed in detail, along with sustainability assessments that show the process to have a relatively low impact on the environment.