The British Association of Play Therapists (BAPT) provides a definition of play therapy (PT) that emphasizes humanistic ideals such as using nondirective play techniques to enable the child's inner resources to bring about growth and change. These therapeutic change processes have never been submitted to empirical testing, partly because of the paucity of valid, reliable instruments to assess child psychotherapy process. Identifying empirically supported change processes is crucial to discovering which change processes work for which child. For example, children with Asperger's disorder have deficits in mentalization—the ability to interpret behaviors of self and others as motivated by underlying mental states. Would a mentalization-informed PT approach or a traditional PT approach be more effective in treating such patients? The Child Psychotherapy Q-Set (CPQ) is a 100-item instrument that assesses the processes within a psychotherapy session. Items reflect a wide range of therapist attitudes and behaviors, patient attitudes and behaviors, and interactions between therapist and patient. To characterize PT process, 24 BAPT play therapists were asked to use the CPQ to rate the prototypical PT session based on their knowledge of PT operationalized by the BAPT definition. Findings indicated that according to these 24 raters, a therapist who is sensitive to the child's feelings and level of development characterizes British PT. Two therapists treating a child with Asperger's disorder over 2 years decreased their session adherence to PT while maintaining their session adherence to mentalization processes. The CPQ enables empirical treatment comparisons with the PT definition of American play therapists.