This chapter reviews the foundations of Therapeutic Assessment, including the comprehensive model of Therapeutic Assessment with Children (TA-C), with its phases and steps; how it developed over time; its guiding theoretical frameworks; and research findings. Foundational theories and core concepts that inform Therapeutic Assessment of Children and serve as the basis for developing case conceptualizations are discussed. Examples include Attachment Theory, Trauma and Adverse Childhood Experiences, Family Systems Theory, Diversity and a Multicultural Perspective, Complexity Theory, Narrative Identity, Phenomenological Psychology, Self-verification Theory, Zone of Proximal Development, Control Mastery Theory, Shame, and Object Relations Theory. Levels of Information are discussed as a way to select order and depth of parent feedback. Family systemic patterns or prototypes that are useful in conceptualizing cases are then reviewed, including Scapegoated Child, Sick Child, Orphan Child, and Solomon's Child. The chapter ends with a review of the research findings on the efficacy and feasibility of TA-C.