The narrow host range of HCV infection has limited animal studies to chimpanzees (Section 1.2), even though numerous attempts have been made experimentally to infect a wide variety of both laboratory and wild animals (reviewed in Bradley, 2000), including several primates other than chimpanzees (Abe et al., 1993). The success in transmitting HBV to primates other than chimpanzees prompted additional attempts with HCV. In this regard, several reports showed successful transmission of HCV to tamarins (Dienes, Feinstone & Popper, 1980; Feinstone et al., 1981; Watanabe et al., 1987), although their lower susceptibility to the virus, combined with the variable incubation periods of infection in different animals (Feinstone et al., 1981), has made this primate model less attractive than the chimpanzee for further development and application. Recently, HCV has been successfully transmitted to tree shrews, which are plentiful and much less expensive than chimpanzees (Xie et al., 1998). Although transient viremia, seroconversion to anti-HCV, and mild hepatitis were observed in many tree shrews, it was not clear whether the liver disease was associated with HCV infection. Much more work needs to be done on this interesting model to assess its future utility properly. As outlined in Sections 1.2 and 1.4, chimpanzees are readily infected with HCV, and their use was central to the discovery of this virus.