Solid lipid nanoparticles (SLNs) have gained considerable attention as advanced nanocarrier systems due to their ability to overcome major limitations associated with conventional drug delivery, particularly poor drug solubility, instability, and low bioavailability. SLNs are composed of physiologically acceptable solid lipids stabilized by surfactants, offering a biocompatible, safe, and versatile platform for pharmaceutical applications. This review provides a comprehensive overview of SLNs, focusing on their composition, preparation techniques, characterization methods, and critical formulation challenges. Widely employed preparation approaches, including high-pressure homogenization, solvent evaporation, microemulsion, solvent injection, and ultrasonication methods, are systematically discussed with emphasis on their advantages and limitations. Furthermore, key evaluation parameters such as particle size, zeta potential, drug loading, crystallinity, in vitro release behavior, stability, and sterility are summarized. The review highlights recent advances in SLN applications across multiple administration routes, including oral, parenteral, transdermal, and intranasal delivery, demonstrating their potential to enhance drug bioavailability, targeting efficiency, and therapeutic outcomes. Despite their promising attributes, challenges related to large-scale production, polymorphic transitions, and long-term stability remain. Overall, this review concludes that SLNs represent a promising and adaptable drug delivery system, with ongoing technological advancements expected to facilitate their successful translation into clinical and industrial applications.