Play tasks that use standardized procedures and materials are a practical way to assess parenting skills, child behaviors, and the ways in which parents and children interact. We describe a systematic process for developing the parent–child play task (PCPT) to assess mother–child interactions for a randomized controlled trial of a video-based parenting program. Participants were 307 mothers and their 3- to 6-year-old children who presented oppositional and disruptive behavior challenges. The validity of the PCPT was investigated by testing (a) the extent to which the tasks elicited the specific parent and child behaviors of interest, (b) the consistency of individuals’ behavior across the play tasks, and (c) the concurrent associations of the PCPT-observed child behaviors and mother reports of child behavior. The different tasks elicited the mother and child behaviors that they were designed to elicit. Behavior consistency across tasks for individual mothers and children was fair to good, with the exception of 2 task-specific behaviors. Mother’s guidance (provision of instructions to foster a skill) during the teaching task and children’s interruptions while mother was busy during the questionnaire task were highly task specific. Modest associations were found between observed children’s noncompliance and inappropriate behaviors and mother-reported conduct problems and oppositional behaviors. Implications for clinical and research assessments are discussed.