This article, the 3rd in a series of 5, introduces the conceptual framework for thematic mapping, a novel approach to case conceptualization. The framework is transtheoretical in that it is not constrained by the tenets or concepts of any one therapeutic orientation and transdiagnostic in that it conceptualizes clients outside the constraints of diagnostic criteria. Thematic mapping comprises 4 components: a definition, foundational principles, defining features, and core concepts. These components of the framework, deemed building blocks, are explained in this article. Like the foundation of any structure, the heuristic value of the method requires that the building blocks have integrity, coherence, and sound anchoring. We assert that the conceptual framework provides a solid foundation, making thematic mapping a potential asset in mental health treatment.