The pharmaceutical evolution of Transdermal Drug Delivery Systems (TDS) has shifted from basic nicotine replacement therapies to complex, multi-layered platforms delivering high-potency analgesics, hormones, and CNS-active agents. As "drug-device combination products," transdermal patches demand a rigorous characterization framework that satisfies both Chemistry, Manufacturing, and Controls (CMC) documentation and the strict bioequivalence (BE) mandates of the US FDA. This manuscript provides an extensive technical exploration of Critical Quality Attributes (CQAs), ranging from mechanical adhesive performance—evaluated through peel, tack, and shear testing—to biopharmaceutical flux evaluation using human cadaver skin models. We detail the practical execution of In Vitro Release Testing (IVRT) and In Vitro Permeation Testing (IVPT), emphasizing the "weight of evidence" approach necessary for generic product approval. Furthermore, the report addresses formulation complexities such as drug crystallization, cold flow, and the impact of chemical enhancers on skin barrier disruption. By synthesizing current regulatory guidance, laboratory examples (e.g., Fentanyl, Lidocaine), and troubleshooting strategies, this text serves as a definitive roadmap for pharmaceutical scientists aiming to align formulation rheology with skin permeation kinetics.