The growing need for sustainable energy sources has driven the extensive exploration of energy conversion technologies for several decades. Electrocatalytic water splitting has been widely considered to play a critical role in sustainable energy conversion, meeting the strong energy demands at a low environmental cost by converting electricity into abundant clean fuel, hydrogen. To overcome the thermodynamic uphill and sluggish kinetics of cathodic hydrogen evolution reactions (HER) and anodic oxygen evolution reactions (OER), the promising nanomaterials catalysts with required properties, most importantly high efficiency and durability, are in urgent need. The performance of nanoscale electrocatalysts relies on their intrinsic nature and structure, both of which can be rationalized by the proper design strategies. However, precise manipulation of the nanomaterial design to get the optimal performance is still a great challenge. In this chapter, we will (i) summarize the fundamentals and mechanisms of HER and OER, (ii) systematically clarify the relationship between electrode materials and catalytic activity and the design principles of catalysts, and (iii) report the recent progress of the state-of-the-art transition metal-based nanomaterials and nanocomposites applied in electrocatalytic water splitting. The emerging materials discovery platforms that facilitate the finding of new catalytic system are also discussed. This chapter intends to broaden the understanding of electrocatalytic water splitting and introduce the functional nanomaterials utilized in this application. Future challenges and outlook of electrocatalytic water splitting are provided at the end to guide the readers who are interested in this promising research field.