Density functional theory (DFT) plays a crucial role in understanding surface chemistry and catalysis. This review discusses recent advances in the theoretical and experimental understanding of reactivity trends at transition-metal surfaces, emphasizing the importance of coupling theory with experiments. Surface chemistry is complex due to the interface between solid and liquid/gas phases, and DFT helps in understanding chemical reactions at metal surfaces, where electron conservation rules differ from gas-phase reactions. Surface chemical reactions are essential for various phenomena, including heterogeneous catalysis, which is vital for industrial processes. The development of sustainable energy solutions relies heavily on heterogeneous catalysis, as it enables the conversion of solar energy into chemical fuels. Efficient catalysts are essential for this process, but their development is limited by the lack of efficient and economically viable materials. The d-band model is a key concept in understanding bond formation at transition-metal surfaces. It explains trends in reactivity and the effects of alloying, structure, and defects. DFT calculations have been used to verify the d-band model and explain experimental results. The model has been applied to various reactions, including methanation, where the activity of a catalyst is determined by the adsorption energies of carbon and oxygen. The paper also discusses the development of descriptors for catalytic activity, which allow for the prediction of catalyst performance based on electronic structure properties. These descriptors are used to identify new catalysts with high activity and selectivity. For example, the Ni-Fe catalyst has been identified as a cheaper alternative to Ni for methanation. The paper highlights the challenges in theoretical surface reactivity and heterogeneous catalysis, including the need for more accurate DFT methods and the development of efficient computational tools. It also emphasizes the importance of experimental validation and the need for a structured database of simulated materials properties to improve reproducibility and facilitate the discovery of new catalysts. Overall, the paper underscores the importance of combining theoretical and experimental approaches in the development of new catalysts and the challenges in achieving this goal. The future of catalysis lies in the integration of computational methods with experimental techniques to design more efficient and sustainable catalysts.Density functional theory (DFT) plays a crucial role in understanding surface chemistry and catalysis. This review discusses recent advances in the theoretical and experimental understanding of reactivity trends at transition-metal surfaces, emphasizing the importance of coupling theory with experiments. Surface chemistry is complex due to the interface between solid and liquid/gas phases, and DFT helps in understanding chemical reactions at metal surfaces, where electron conservation rules differ from gas-phase reactions. Surface chemical reactions are essential for various phenomena, including heterogeneous catalysis, which is vital for industrial processes. The development of sustainable energy solutions relies heavily on heterogeneous catalysis, as it enables the conversion of solar energy into chemical fuels. Efficient catalysts are essential for this process, but their development is limited by the lack of efficient and economically viable materials. The d-band model is a key concept in understanding bond formation at transition-metal surfaces. It explains trends in reactivity and the effects of alloying, structure, and defects. DFT calculations have been used to verify the d-band model and explain experimental results. The model has been applied to various reactions, including methanation, where the activity of a catalyst is determined by the adsorption energies of carbon and oxygen. The paper also discusses the development of descriptors for catalytic activity, which allow for the prediction of catalyst performance based on electronic structure properties. These descriptors are used to identify new catalysts with high activity and selectivity. For example, the Ni-Fe catalyst has been identified as a cheaper alternative to Ni for methanation. The paper highlights the challenges in theoretical surface reactivity and heterogeneous catalysis, including the need for more accurate DFT methods and the development of efficient computational tools. It also emphasizes the importance of experimental validation and the need for a structured database of simulated materials properties to improve reproducibility and facilitate the discovery of new catalysts. Overall, the paper underscores the importance of combining theoretical and experimental approaches in the development of new catalysts and the challenges in achieving this goal. The future of catalysis lies in the integration of computational methods with experimental techniques to design more efficient and sustainable catalysts.