• Multi-step thermodynamics. The thermodynamics of an elementary step are determined by binding energies of adsorbed intermediates involved in this step. Past approaches have correlated the overall activity and selectivity of a specific reaction with the binding energies of key intermediates, which are readily calculated using first-principles based methods. This line of research leads to the development of tools for screening catalysts, employing for instance the d-band model or the generalized coordinate number. • Multi-step kinetics. Beyond thermodynamics, kinetic parameters, including but not limited to activation barriers, transfer coefficients and preexponential factors, are important for a quantitative understanding of electrocatalysis. In a few recent reports, these kinetic factors have been shown to change the qualitative trend of activity. For instance, in a microkinetic analysis accounting for activation barriers and transfer coefficients of elementary steps, the peak of the volcano plot on the binding energy axis changes with electrode potential. In the quest to simplify microkinetic analyses, it has become a customary practice to identify a single step that governs the overall rate of the reaction, termed the rate-determining step (RDS). The transition of the RDS from one step to another is often regarded as the cause of potential-dependent Tafel slopes. • Electric double layer effects. Electrocatalytic reactions take place in the electric double layer (EDL) at the catalyst-electrolyte interface. There exist many EDL effects, including the classical Frumkin corrections (i.e., the effects on potential and reactant concentration at the most probable reaction plane), field-dependent adsorption energies of intermediates, and dependency of the solvent reorganization energy on the surface charge density. In addition to these equilibrium EDL effects, nonequilibrium EDL effects, firstly proposed by Levich et al. in the 1950s, are resurfacing in recent studies. • Mass transport. The consumption (production) of reactants (products) significantly influences local reactant/product concentrations and pH in the near-surface region. Recent progress has enabled direct probes of changes in ion concentrations and pH with a spatial resolution down to a few hundreds of nanometers. We categorize existing theoretical methodologies for modeling electrocatalytic reactions into four levels, as summarized in Figure . Level 1 (L1) considers only thermodynamics. Specifically, L1 models focus on the potential energy profiles of electrocatalytic reactions with the binding energies of intermediates that can be readily calculated from density functional theory (DFT)based first-principles simulations. Despite the simplicity, L1 models can explain, surprisingly well, overall trends of activity and selectivity within groups of catalysts with similar electronic structures. The success of these models is evident from effective and easy-to-implement tools for screening catalyst materials, encompassing the d-band model, the generalized coordinate number, and volcano plots. In view of the simplicity of L1 models, it is unsurprising that counterexamples have been reported in the literature. For instance, the volcano plot predicts a sequence Pt(111) > Pt(100) > Pt(110) > Pt(211) for the oxygen reduction reaction (ORR), yet experiments show a trend Pt(211) > Pt(110) > Pt(111) > Pt(100). Additionally, concerns have been raised that the thermodynamics-based method may yield inaccurate results, due to the neglect of kinetic parameters of elementary steps, e.g., activation barriers and transfer coefficients. As an obvious step, kinetic factors are incorporated into L1 models on level 2 (L2). L2 models treat the kinetics on two sublevels. The first sublevel relies on the RDS concept, and only the kinetics of the RDS is considered. Practically, the RDS is often identified based on Tafel slope analysis that is, however, problematic. Values of Tafel slope can vary to a great extent among different measurements, leading to disparate reaction mechanisms reported in different studies. Furthermore, the Tafel slope exhibits high sensitivity to adsorbate coverages. Therefore, the RDS usually changes with electrode potential. These inconsistencies necessitate a full microkinetic model that treats the thermodynamics and kinetics of all elementary steps without singling out an RDS, constituting the second sublevel on the L2. These models have been utilized to rationalize potential-dependent Tafel slopes and volcano plots for various reactions. Recent years have witnessed a growing awareness of the significance of electrolyte composition.