Smart nanocarriers represent a paradigm shift in pharmaceutical nanotechnology, enabling precision drug delivery through stimuli-responsive mechanisms and targeted therapeutic interventions. These intelligent nanosystems respond to endogenous physiological stimuli such as pH gradients, redox potential, enzyme activity, and temperature variations, or exogenous triggers including light, magnetic fields, and ultrasound, facilitating controlled and site-specific drug release. This article comprehensively reviews the design principles, physicochemical characteristics, and targeting strategies of smart nanocarrier platforms, including polymeric micelles, liposomes, dendrimers, mesoporous silica nanoparticles, and hybrid nanosystems. Emphasis is placed on the molecular mechanisms underlying passive and active targeting, stimuli-responsive drug release pathways, and their integration into precision nanomedicine. The therapeutic applications of smart nanocarriers in oncology, inflammatory disorders, infectious diseases, and chronic pathological conditions are critically examined, highlighting their potential to overcome multidrug resistance, reduce systemic toxicity, and enhance therapeutic efficacy. Despite significant advances, clinical translation faces formulation complexity, biological barriers, scalability concerns, and regulatory challenges. Future directions include the development of multi-stimuli-responsive systems, personalized nanomedicine platforms, and next-generation theranostic nanocarriers combining diagnostic and therapeutic functionalities for improved patient outcomes in precision medicine.