Oxygen-deficient/non-stoichiometric tungsten oxides are a distinct class between two only stoichiometric tungsten oxides, i.e., WO3 and WO2. The WO3 could be termed as a parent structure while understanding the non-stoichiometric tungsten oxides. The oxygen vacancies (O-vacs) induced in the parent (WO3) structures cause the re-adjustment of atomic arrangement to compensate for the oxygen deficiency and follow the crystal chemistry. The degree of O-vacs determines the range of WO3-x. The W18O49 (WO2.722) is the highest oxygen-deficient stable phase. These non-stoichiometric oxides proved superior to WO3 for various applications. Thus, the composition, chemistry, crystallography, synthesis, and formation mechanisms of such materials have been elaborated. Further, the superiority of the materials in view of few applications is well discussed. Finally, the importance of non-stoichiometry/O-vacs in metal oxides for various functional applications can be understood. [GRAPHICS] .