Producing hydrogen fuel through environmentally friendly electrochemical and solar-driven photoelectrochemical (PEC) water splitting is a very promising approach for providing affordable clean energy. The scalable and sustainable production of hydrogen demands efficient and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) beyond platinum and other precious-metal catalysts. This review provides an overview of molybdenum disulfide (MoS2) and related compounds as inexpensive alternative electrocatalysts for HER catalysis and PEC water splitting. After a background introduction, we discuss the important approaches to improving the intrinsic catalytic activity and overall catalytic performance of MoS2. We further review the key developments in combining MoS2 with semiconductors for integrated PEC systems for direct solar-to-fuel conversion and provide insights on how to design efficient solar-driven water-splitting systems. Our perspectives on the key challenges and future directions for development of earth-abundant HER electrocatalysts and PEC water splitting are also discussed. Display omitted Generating hydrogen fuel through electrochemical and solar-driven photoelectrochemical (PEC) water splitting is a very promising approach to providing affordable clean energy, reducing our reliance on fossil fuels, and mitigating the impact of climate change. The sustainable production of hydrogen demands efficient and robust earth-abundant catalysts that are not based on platinum and other precious metals for the hydrogen evolution reaction (HER). This review focuses on molybdenum disulfide (MoS2) and related compounds as inexpensive alternatives for HER catalysis and PEC water splitting. We discuss key approaches to improving the intrinsic catalytic activity and overall catalytic performance and the developments in combining MoS2 with semiconductors to realize solar-to-fuel conversion. We also discuss how to design efficient PEC water-splitting systems, as well as our perspectives on the key challenges and future directions for developing earth-abundant HER electrocatalysts and PEC water splitting. This review provides an overview of molybdenum disulfide (MoS2) and related compounds as inexpensive alternatives for hydrogen evolution reaction electrocatalysis and solar-driven photoelectrochemical water splitting toward the goals of producing hydrogen fuel to provide affordable clean energy. Important approaches to improving the intrinsic catalytic activity, developments in combining MoS2 with semiconductors to design efficient water-splitting systems, and perspectives on the challenges and future directions are discussed.