Drug disposition is highly regulated by membrane transporters. Some transporter-mediated drug-drug interactions (DDIs) may not manifest themselves in changes in systemic exposure but rather in changes in tissue exposure of drugs. To better assess the impact of transporter-mediated DDIs in tissues, positron emission tomography (PET)-a noninvasive imaging method--plays an increasingly important role. In this article, we provide examples of how PET can be used to assess transporter-mediated DDIs in different organs.