The discovery that RNA can act as a biological catalyst, as well as a genetic molecule, indicated that there was a time when biological reactions were catalysed in the absence of protein-based enzymes. It also provided the platform to develop those catalytic RNA molecules, called ribozymes, as trans -acting tools for RNA manipulation. Viral diseases or diseases due to genetic lesions could be targeted therapeutically through ribozymes, provided that the sequence of the genetic information involved in the disease is known. The hammerhead ribozyme, one of the smallest ribozymes identified, is able to induce site-specific cleavage of RNA, with ribozyme and substrate being two different oligoribonucleotides with regions of complementarity. Its ability to down-regulate gene expression through RNA cleavage makes the hammerhead ribozyme a candidate for genetic therapy. This could be particularly useful for dominant genetic diseases by down-regulating the expression of mutant alleles. The group I intron ribozyme, on the other hand, is capable of site-specific RNA trans -splicing. It can be engineered to replace part of an RNA with sequence attached to its 3' end. Such application may have importance in the repair of mutant mRNA molecules giving rise to genetic diseases. However, to achieve successful ribozyme-mediated RNA-directed therapy, several parameters including ribozyme stability, activity and efficient delivery must be considered. Ribozymes are promising genetic therapy agents and should, in the future, play an important role in designing strategies for the therapy of genetic diseases.