Flow biocatalysis uses enzymes to catalyse chemical reactions under continuous flow with applications in the synthesis of fine chemicals, pharmaceuticals, agrochemicals and natural products, among others. The flow set-up, which, in its core elements, is no different from a classical flow chemistry apparatus, has enabled reaction conditions and reactivities impossible to achieve in batch mode. In this Primer, we explain the underlying principles of flow biocatalysis and guide the reader on how to approach the field efficiently. The unique requirements for the catalyst preparation are addressed. Different set-ups and common variations are discussed, allowing users to decide which is appropriate for their specific application. Through a hypothetical case study where a redox enzymatic reaction is conducted in flow, we review the typical data obtained and guide the reader towards the best reaction conditions. We next outline data-handling and process-control approaches specific to flow biocatalysis, illustrating current efforts towards community-wide reporting standards that will improve reproducibility. Finally, we examine field-specific challenges —including catalyst deactivation in biphasic streams, fouling-induced pressure spikes and support disposal — and propose future directions such as magnetically stabilized beds, three-dimensional-printed bespoke reactors and self-optimizing platforms that promise to expand the reach and industrial uptake of flow biocatalysis.