Antisense oligodeoxynucleotides (ODNS) can be used to specifically inhibit hepatitis C viral gene expression. Due to its high degree of conservation and its important function as internal ribosomal entry site, the 5’-non-coding region of the hepatitis C virus has been the most effective target to inhibit translation so far. Inhibition of luciferase reporter gene expression of up to 96 ± 2% has been achieved. Modifications of ODNs like phosphorothioate, methylphosphonate or benzylphosphonate modification of terminal or intramolecular internucleotide phosphates lead to altered lipophilicity and binding stability to its RNA target and resistance against serum nucleases. The mode of action of ODNs is mainly dependent on an efficient induction of RNase H activity. The uptake of ODNs occurs via receptor-mediated or absorptive and fluid-phase endocytosis. After release from the endosomes, ODNs may exert their effects by interaction with cytosolic or nuclear structures. Side effects can occur when this interaction affects intra- or extracellular targets essential for biological cell function. If these problems can be solved, antisense technology has the potential for future therapy of human disea