The state-of-the-art strategies for perfusion process design, development, and optimisation are discussed. We first introduce the essential steps and boundaries for the development of a perfusion process. Next, we evaluate clone and media screening for perfusion processes and compare various alternative scale-down systems. Once the expression system is selected, we proceed with the design of the reactor operating conditions to maximise volumetric productivity and yield while reducing medium consumption. The fundamental issue of product quality attributes for biologics and biosimilars, and the impact of perfusion operating mode are also discussed. Finally, we introduce the main objectives in the scale-up of the developed process to clinical and commercial productions.