Multiplayer online games are engineered to be highly engaging. Their success is reflected in the nearly 20 million participants, including roughly 5 million adolescents and children, who often play as much as 20 hours per week. A central question for this paper is whether players are deeply engaged in the positive sense of “flow” and optimal experience, or in the pathological sense of “addiction”—or both. The answer appears to be related to what individuals bring to the game. Those with low self-esteem and/or real world problems may be particularly vulnerable to negative effects. The paper invokes motivational theories to frame the dynamics of such effects, considers age differences in such dynamics, and develops directions for further research.