Continuous-flow hydrothermal and solvothermal synthesis methods (CHS and CSS) have their origin in the early 1990s with the pioneering work of Prof. Adschiri exploiting the properties of supercritical water to achieve the generation of a range of binary metal oxide nanoparticles in a flow system. Since its inception, a great deal of work on CHS has focussed on overcoming the physical and engineering challenges it presents. Just as these engineering challenges were overcome, so were the new chemical obstacles raised by the technique surmounted. From its beginnings as a novel route to binary metal oxide nanocrystals, the CHS/CSS method now offers a clean and scalable synthetic process for the production of a vast array of (nano)materials, including more complex tertiary and ternary metal oxides, metal sulphides, metal phosphates, metals, layered double hydroxides, and even porous materials such as zeolites and metal organic frameworks. Here, we review the progress made in expanding the scope of the CHS method to these various classes of materials, with a focus on the chemical challenges, and opportunities, presented by continuous-flow hydrothermal and solvothermal techniques.