Transdermal drug delivery systems (TDDS) are modern pharmaceutical technologies designed to deliver drugs across the skin into systemic circulation in a controlled manner. This approach avoids the limitations of conventional oral therapy, such as first‑pass metabolism and frequent dosing, thereby improving therapeutic efficiency and patient adherence. Transdermal patches allow continuous drug release, minimize side effects, and provide a non‑invasive route of administration. This review focuses on the essential constituents of transdermal patches, including polymer matrices, active drugs, permeation enhancers, adhesives, backing layers, release liners, plasticizers, solvents, and carrier systems like liposomes. Different patch designs such as drug-inadhesive, reservoir, matrix, and vapor patches are explained along with the pathways of drug transport through the skin, namely transepidermal and follicular routes. In addition, the biological, physicochemical, and environmental variables affecting skin permeation are discussed. Common manufacturing techniques and evaluation parameters such as thickness, weight variation, drug content uniformity, mechanical strength, moisture studies, and in‑vitro and in‑vivo performance tests are also summarized. Overall, TDDS provide a reliable and effective platform for sustained drug delivery and represent a promising strategy for future pharmaceutical development involving both lipophilic and hydrophilic drugs.