Recent advances in molecular biology have led to the development of novel small molecules that target specific viral proteins of the hepatitis C virus (HCV) life cycle. These drugs, collectively termed directly acting antivirals (DAA) against HCV, include a range of non-structural (NS) 3/NS4A protease, NS5B polymerase, and NS5A inhibitors at various stages of clinical development. The rapid replication rate of HCV, along with the low fidelity of its polymerase, gives rise to generations of mutations throughout the viral genome resulting in remarkable sequence variation in the HCV population, known as a quasispecies. The efficacy of DAAs is limited by the presence of those mutations that give rise to amino-acid substitutions within the targeted protein, and that affect the viral sensitivity to these compounds. Thus, due to the high genetic variability of HCV, variants with reduced susceptibility to DAA can occur naturally even before treatment begins, but usually at low levels. Not surprisingly then, these changes are selected in patients either breaking through or not responding to potent DAA treatment. In vitro or in vivo, six major position mutations in the NS3 HCV protease (36, 54, 155, 156, 168, and 170) have now been reported associated with different levels of resistance. The amino acid composition at several of the drug resistance sites can vary between the HCV genotypes/subtypes, resulting in different consensus amino acids leading to a reduction in replicative fitness as well as reduced DAA sensitivity. Different amino acid diversity profiles for HCV genotypes/subtypes suggest differences in the position/type of immune escape and drug resistance mutations. Also, different pathways of resistance profiles based on the chemical scaffold (linear or macrocyclic) of the protease inhibitors have been described. This review first describes how resistance to a protease inhibitor can develop and then provides an overview of the mechanism of how particular mutations confer varying levels of resistance to protease inhibitor, which have been identified and characterized using both genotypic and phenotypic tools. Future potential therapeutic strategies to assist patients who do develop resistance to protease inhibitors are also outlined. The challenge developing new HCV protease inhibitors should take into consideration not only the antiviral potency of the drugs, the occurrence and importance of side effects, the frequency of oral administration, but also the resistance profiles of these agents. Copyright © 2011 European Association for the Study of the Liver. All rights reserved.