Single-use (SU) systems are now in common use in pharmaceutical bioprocessing, as well as in other related technologies such as the manufacture of diagnostics and other biological products, and their popularity is increasing. Some types of SU systems have been in use for many decades now. The earliest SU systems being disposable filter cartridges that do not require a stainless steel (SS) filter housing. This present article seeks to focus in particular on SU bioreactors for cell culture and bacterial fermentation for the purpose of producing therapeutic proteins, monoclonal antibodies and vaccines. SU bioreactors are of particular value in early phase (Clinical Phases 1 and 2) GMP manufacturing. In some cases their use has now stretched through into commercial processing, albeit that the scale of operation is currently limited and in general the largest commercially available SU bioreactors are around 2000L working volume (WV). However, the small footprint that they require, and the reduction in investment needed for support services and utilities, means that the scale limitations can be overcome to a significant degree by having multiple SU bioreactors operating in parallel within a facility. The harvest from multiple bioreactors can be pooled for downstream processing, or each harvest can be processed as a separate batch, based upon considerations of the risks versus the economies of pooling.