Low-dimensional materials and their hybrids have emerged as promising candidates for electrocatalytic and photocatalytic hydrogen evolution and CO₂ conversion into useful molecules. Progress in synthetic methods for the production of catalysts coupled with a better understanding of the fundamental catalytic mechanisms has enabled the rational design of catalytic nanomaterials with improved performance and selectivity. In this Review, we analyse the state of the art in the implementation of low-dimensional nanomaterials and their van der Waals heterostructures for hydrogen evolution and CO₂ reduction by electrocatalysis and photocatalysis. We explore the mechanisms involved in both reactions and the different strategies to further optimize the activity, efficiency and selectivity of low-dimensional catalysts.