Recent theoretical and experimental work has advanced our understanding of the meta­bolic pathways and elucidated the intrinsic metabolic potential and kinetic limitations of microbial conversion of glycerol to 1,3-propanediol by the major strains isolated. The maximum product yield conceivable is about 0.72 mol/mol in fermentation with glyce­rol as a sole carbon and 1.0 mol/mol in fermentation with cosubstrates. However, these high yields are normally not achieved in fermentations with high final product concen­tration which is kinetically constrained to about 65 -70 g/1. Further work is needed to optimize the distribution of reducing power released from the metabolism of glycerol and/or cosubstrate and to reduce the formation of toxic by-products for simultaneously achieving a high PD yield and concentration. This may be best achieved by re-de­signing the metabolic pathway and properly controling the cultivation conditions. Pro­gress has already been made in these directions. The use of recombinant techniques also opens new ways for producing PD from other cheap carbon sources. It will become mo­re evident that the microbial production of 1,3-propanediol is technically more flexible and economically more competitive than the chemical route.