The need to provide effective mental health treatments for adolescents has been described as a global public health challenge [27]. In this paper we discuss the exploratory evaluations of the first adolescent intervention to fully integrate a computer game implementing Cognitive Behavioural Therapy. Three distinct studies are presented: a detailed evaluation in which therapists independent of the design team used the game with 6 adolescents experiencing clinical anxiety disorders; a study in which a member of the design team used the game with 15 adolescents; and finally a study assessing the acceptability of the game and intervention with 216 practicing therapists. Findings are presented within the context of a framework for the design and evaluation of complex health interventions. The paper provides an in-depth insight into the use of therapeutic games to support adolescent interventions and provides stronger evidence than previously available for both their effectiveness and acceptability to stakeholders.