This study considered whether the distinction between core and peripheral criteria for behavioral addiction, previously drawn with respect to computing activities in general, applies in the specific area of Massively Multiplayer Online Game playing. Questionnaire items were administered over the Internet to 442 game players. Factor-analysis of the data supported the previous findings for computing in general. An addiction factor loaded on items tapping previously identified core criteria (conflict, withdrawal symptoms, relapse and reinstatement and behavioral salience) and a (non-pathological) engagement factor loaded on items tapping previously identified peripheral criteria (cognitive salience, tolerance and euphoria). Analysis of response frequencies supported the existence of a developmental process whereby peripheral criteria are met before core criteria. Players who might be considered addicted using a monothetic classification system involving only the core criteria were shown to spend a significantly greater amount of time playing per week than those endorsing only the peripheral criteria. It is concluded that the study supports the idea that it is inappropriate to use some of the previously used criteria for addiction when researching or diagnosing computer-related addictions. Implications of the present findings for Internet-mediated data collection methodologies are also discussed.