A modular flow reactor is formed of a plurality of modules, wherein each module comprises a body having at least one conduit passing therethrough, and wherein a plurality of said modules are aligned along a longitudinal axis such that said conduits of said modules are aligned to form a passage for fluid, wherein each module has a length along said longitudinal axis which is less than the length of the module perpendicular to the longitudinal axis. The modules are “slices” rather than “tubes” and a plurality of said modules can be aligned linearly so that the conduits form a tube.