The growing demand for high-speed, miniaturized, and energy-efficient photonic devices has driven intense research into materials with strong and tunable nonlinear optical (NLO) properties. Tin dioxide (SnO2), a wide-bandgap n-type semiconductor, has recently attracted significant attention in this context due to its intrinsic chemical stability, optical transparency, and ease of structural and compositional tailoring. This chapter presents a comprehensive review of the third-order nonlinear optical response of SnO2 nanostructures, including phenomena such as two-photon absorption (TPA), reverse saturable absorption (RSA), optical Kerr effect, and third-harmonic generation (THG). The impact of various physical and chemical factors such as particle size, morphology, crystallinity, doping (with rare-earth and transition metals), and surface functionalization on the NLO performance of SnO2 is critically examined. Furthermore, the role of host matrices, including sol–gel glasses, polymer films, and liquid suspensions, in modulating the NLO behaviour is discussed, highlighting how matrix–nanoparticle interactions affect field enhancement, carrier dynamics, and thermal effects. State-of-the-art experimental techniques such as Z-scan, pump–probe spectroscopy, and THG measurements are described, along with key findings from recent literature. Applications in optical limiting, switching, and photonic integration are also addressed, underscoring the technological relevance of SnO2 nanostructures in next-generation optical systems.