Electrocatalytic properties of materials are governed by the electronic structure, stability, and reactivity of the surface layer which is exposed to the electrolyte. Over the years, different strategies have been developed to tailor electrocatalyst surfaces but also to reduce the cost of these materials, which is the bottleneck for any practical application. When a very thin metallic layer, intended to serve as an electrocatalyst, is placed over a substrate, its configuration is influenced by the structure of the substrate due to lattice mismatch, while the electronic structure is affected due to the strain and the electronic effects of the support. This results in altered bonding within the electrocatalyst layer and the modification of its electronic properties when compared to the pure phase. In this contribution, we address the possibilities of theoretical prediction of surface properties of atomically thin electrocatalyst films formed over different substrates, focusing on the metal side of the electrified interface. While all these properties can be calculated quite easily using modern computational techniques (but used with care), most often based on density functional theory, we also address an attractive, fast screening possibility to estimate the properties of monometallic and multimetallic overlayers using small sets of calculations on model systems. We discuss how lattice mismatch between a substrate and an overlayer can be used to predict the properties of electrocatalytic films, limitations of such approach, and a possibility of deploying of large databases which enable rapid prescreening of different support/overlayer systems for various electrocatalytic applications.