In the recent years a lot of attention has been paid to hairy root culture as a promising strategy to produce a variety of secondary metabolites including pharmaceuticals, pigments and also fragrances. Hairy roots are tumorous outgrowth developing at the site of infection of wounded plant parts with gram-negative soil bacteria Agrobacterium rhizogenes. During the infection process a part of root-inducing (Ri) plasmid from bacteria is transferred to the plant cell where it integrates into the genome. An increasing number of hairy root cultures is known to produce secondary metabolites in quantities comparable with those in intact plant roots. Some secondary metabolites occurring only in the aerial parts of the intact plants were also found in the hairy roots of some species. So far, production of secondary metabolites in hairy root cultures of about 100 dicotyledonous species and some gymnosperms has been reported. The suitability of hairy root cultures for bioprocessing can be attributed to their genetic and biochemical stability and rapid growth in hormone-free media. Hairy root cultures of medicinal plants and factors influencing their productivity will be reviewed.