Internet addiction (IA) has been explored worldwide and involves some features that might harm social, psychological, and functional aspects. The vulnerability of adolescents might result in some addictive behaviors, and the Internet is one of them. The aim of this systematic review was to identify empirical studies that have been done to explore instruments that have been used to assess IA and to verify what variables and comorbidities are related to IA in adolescents, including executive functions, such as attention. To accomplish our aim, a systematic search was conducted in the following databases: PubMed, Web of Science, and PsycINFO. Some inclusion and exclusion criteria were used to select the studies and 44 in total were analyzed independently. The results showed that the most used instrument to assess IA is the Young's Internet Addiction Test. A few studies have been developed in the Western world, and most of them have analyzed attention bias in attention-deficit/hyperactivity disorder (ADHD) and non-ADHD groups. Furthermore, other variables have been widely explored in the studies, such as depression, sleeping patterns, body weight, aggressiveness, and other addictive substances.