Widespread use of embryonic and adult stem cells for therapeutic applications will require reproducible production of large numbers of well-characterized cells under well-controlled conditions in bioreactors. During the past two years, substantial progress has been made towards this goal. Human mesenchymal stem cells expanded in perfused scaffolds retained multi-lineage potential. Mouse neural stem cells were expanded as aggregates in serum-free medium for 44 days in stirred bioreactors. Mouse embryonic stem cells expanded as aggregates and on microcarriers in stirred vessels retained expression of stem cell markers and could form embryoid bodies. Embryoid body formation from dissociated mouse embryonic stem cells, followed by embryoid body expansion and directed differentiation, was scaled up to gas-sparged, 2-l instrumented bioreactors with pH and oxygen control.