The new direct-acting antiretroviral drugs for treating hepatitis C have generated considerable momentum for treating HCV infection among people who inject drugs and perhaps ‘ending the HCV epidemic’. There are, however, important epidemiological and behavior research problems that need to be addressed before ‘ending the HCV epidemic’ will be possible. The development of direct-acting antivirals (DAAs) that cure HCV infection in more than 90% of patients with minimal side effects has led to calls for ‘eliminating HCV infection’ or ‘ending HCV epidemics’ among people who inject drugs (PWID) 1. Some of the calls for eliminating HCV infection have included substantial allocations of new funds, e.g. New York State 2. Grebely et al. 3 have estimated the number and prevalence of people with a recent history (within past year) of injecting drug use who are living with hepatitis C virus (HCV) viremia and the proportion of people with recent injecting drug use among all people living with HCV infection at the global, regional and national levels. A major value of this study is its potential contribution as a baseline for ‘eliminating HCV’. While DAAs and the lessons from successfully controlling HIV among PWID in many areas 4 should certainly be useful in controlling HCV among PWID, the data presented in Grebely et al. illustrate that there is still a major amount of epidemiological and behavioral science that will be needed before ‘eliminating’ (or even ‘controlling’) HCV among PWID can be accomplished. First, what would ‘elimination/ending the epidemic’ of HCV among PWID look like? From early in the HIV epidemic among PWID, we had multiple examples, e.g. Glasgow, Scotland, Lund, Sweden, Sydney, Australia and Tacoma, WA, USA, where large-scale implementation of syringe access programs have kept HIV prevalence stable at less than 5% 5. The Grebely et al. data do not provide many (if any) examples of areas where HCV viremia has been stabilized at ‘ending the epidemic’ levels. Would 5% viremia in the local PWID population be a realistic goal? What sort of HCV combined prevention and care programs would be needed to stabilize HCV at 5% or less in a PWID population? Such a system would need to detect and treat existing cases of HCV infection as rapidly as new infections were occurring. Secondly, how will we develop better estimates of the size of PWID populations and the percentage of PWID who are HCV viremic? The uncertainty intervals in the Grebely et al. estimates are rather large, typically almost as large as the estimates themselves. Uncertainty in the estimates of the PWID population size and the percentage of PWID who are HCV viremic would create difficult problems for local officials who would have the task of allocating resources and planning logistical operations for rapidly scaling-up treatment for HCV-infected PWID. Thirdly, what new interventions can be developed to reduce HCV transmission behavior among viremic PWID? One of the important factors in HIV prevention was that PWID who learned that they were HIV-seropositive greatly reduced transmission behavior (passing their used needles and syringes to others) well before antiretroviral therapy (ART) was available 6. We have not yet seen the equivalent reductions in transmission behaviors among PWID who know that they are HCV-seropositive 7. How do we successfully encourage HCV viremic PWID to reduce transmission behavior? Fourthly, what interventions can be developed and implemented to reduce the very high HCV incidence rates among people who have recently begun injecting drugs 8? Many new injectors typically do not identify as ‘drug injectors’, and thus do not utilize HIV/HCV safer injection programs 9. Fifthly, what interventions will be implemented to reduce the rates that drug users transition to injecting drug use? There have been calls for more research on this topic 10, and there are current research studies addressing this subject. However, the objective should be to develop an evidence base for reducing initiation into injecting drug use that is comparable to the evidence base for medication assisted treatment and for syringe access programs. The new DAAs certainly give us the capability of treating very large numbers of HCV infected people who use drugs and greatly reduce morbidity and mortality, and programs to provide access to HCV treatment should be scaled-up in high-, middle- and low-income countries as quickly as feasible, and the additional research needed to learn how to ‘end HCV epidemics’ among PWID should be funded. Finally, and perhaps most importantly, while the above may be considered scientific tasks, it is important to emphasize that the research should be conducted in full collaboration with PWID and drug-user organizations. Such collaboration should not only greatly improve the quality of the research, but also greatly improve the likelihood that the findings will be incorporated into public health programs that could ‘end the HCV epidemic’ among people who use drugs. None.