The internet is transforming the nature of social relationships. There is a growing debate among scholars about how the internet can enhance existing relationships or facilitate the development of new relationships, including illicit ones. Research in this field has predominately focused on cybersex, pornography, and social networking sites, with a few studies on internet infidelity. A clear definition of internet infidelity is important theoretically, empirically, and therapeutically; however, research in this area is limited. This chapter explores the different types of infidelity and the challenges in defining internet infidelity. It offers a critical review of the existing literature to make sense of the complexities and challenges in carrying out research on this subject. It also provides recommendations and directions for future research.