Tin oxide (SnO2), also known as stannic oxide, has gathered substantial interest across various scientific disciplines due to its versatile properties and promising applications. It has emerged as a potential semiconductor like TiO2, ZnO, and NiO due to its optical, mechanical, physiological, and electronic properties. This chapter provides an in-depth introduction of SnO2, including phase transformation mechanism, their physical, chemical, optical, mechanical, and thermal properties and further discussion on its multidisciplinary applications. Firstly, different phases of SnO2 with their transitions into one another are discussed. Phase transformation of SnO2 plays an important role in determining its properties and applicability for various applications. SnO2 is an n-type semiconductor with a wide bandgap of ~3.6 eV at room temperature. This wide bandgap leads to its diverse electronic and photonic applications. Furthermore, the physical, chemical, and optical properties of SnO2 are highlighted. The chemical stability and redox behavior make it suitable to undergo reversible oxidation–reduction reaction in catalysis and sensing. Its thermal and chemical stability makes it resilient in harsh environment. At the end, chapter concludes with the diverse applications of SnO2 across fields such as gas sensing, biomedical, optoelectronics, catalysis, and so on. Thus, this chapter layouts the foundational understanding of SnO2 nanoparticles, taking into account of phase transformation, properties optimizations, and multifunctional applications.