Advances in smart materials continue to reshape drug and gene delivery by enabling precise, site-specific therapeutic action. This fully revised 2nd edition provides an authoritative overview of the chemistry, design, and biomedical applications of stimuli-responsive systems, reflecting major breakthroughs over the past five years. The volume comprehensively covers pH-, temperature-, redox-, glucose-, enzyme-, light-, ultrasound-, electrical-, magnetic-, and swelling-responsive platforms, while expanding into emerging areas including multi-stimuli architectures, drug-device combinations, and AI-driven design tools. Nineteen chapters by leading international experts explore material synthesis and characterization, mechanisms of responsiveness, and delivery strategies for small molecules, nucleic acids, proteins, gene therapies, and cell therapies. The edition highlights rapidly growing theranostic applications integrating imaging and treatment, alongside essential considerations for manufacturability, regulatory pathways, and commercialisation. Richly illustrated with preclinical and clinical examples, the book emphasizes how environmentally triggered systems enhance targeting precision, improve therapeutic index, and enable adaptive drug release control.