Hydrogen has been proven to be a super energy carrier for a sustainable energy scheme with a low carbon footprint, earth abundance, and high energy density, and is acknowledged as a major approach to fighting climate change and reducing the environmental pollution. The “water-splitting process” which creates oxygen (O2) and hydrogen (H2) from water is the most advantageous technique for generating renewable hydrogen gas. In this focused chapter, we have briefly explained the methods for water-splitting reactions, such as electrocatalytic (EC) water-splitting, photocatalytic (PC) water-splitting, and photoelectrocatalytic (PEC) water-splitting. The current developments in electrocatalytic or photocatalytic oxygen/hydrogen generation are reviewed after a quick outline of the mechanisms and principles of these technologies. In addition, we also deliberate the recently reported nanocomposites for water-splitting, especially transition metal oxides/carbides (TMOs/TMCs), Z-scheme heterojunction, and metal-organic frameworks (MOFs)-based nanocomposites. Furthermore, a fundamental understanding of the structure-activity relation has been explored. Finally, this research is expected to contribute as a foundation and stepping stone in the development of unique nanocomposites for alternative electricity and an ecologically friendly environment.