Over the last 30 years, expectations for the quality, validity, and objectivity of the outcome measures used to assess the impact of behavior change interventions related to HIV have steadily increased. At this point (mid-2014 at this writing), biologic evidence or biomarkers of the incidence of HIV and other sexually transmitted infections in a target population is clearly preferable to self-reports of behavior. This kind of evidence is, however, much more expensive to collect than participants' reports of behavior change (eg, increased condom use, reduced substance use or abstinence from substance use, and high levels of medication adherence). In addition, although potentially less subject to reporting bias, biomarkers and biologic outcomes have their own flaws. In this article, we review the literature on the validity of self-reports of outcomes most relevant to HIV behavior change interventions, sexual behavior (ever having had sex and condom use), substance use, and medication adherence. We note the extent to which they may be adequate outcome measures without biologic data, and the conditions under which they may be most likely to be sufficient. We also argue, like many others, that where possible, both self-report and biologic measures should be collected.