Gold nanomaterials have diverse applications ranging from healthcare and nanomedicine to analytical sciences and catalysis. Microfluidic and millifluidic reactors offer multiple advantages for their synthesis and manufacturing, including controlled or fast mixing, accurate reaction time control and excellent heat transfer. These advantages are demonstrated by reviewing gold nanoparticle synthesis strategies in flow devices. However, there are still challenges to be resolved, such as reactor fouling, particularly if robust manufacturing processes are to be developed to achieve the desired targets in terms of nanoparticle size, size distribution, surface properties, process throughput and robustness. Solutions to these challenges aremore effective through a coordinated approach from chemists, engineers and physicists, which has at its core a qualitative and quantitative understanding of the synthesis processes and reactor operation. This is important as nanoparticle synthesis is complex, encompassing multiple phenomena interacting with each other, often taking place at short timescales. The proposed methodology for the development of reactors and processes is generic and contains various interconnected considerations. It aims to be a starting point towards rigorous design procedures for the robust and reproducible continuous flow synthesis of gold nanoparticles.