In spite of the strong relevance of reactions in electrocatalysis, in particular for the electrochemical energy conversion and storage, the number of theoretical studies addressing electrocatalytic reactions from first principles is still limited. This is due to the fact that there are two factors adding considerable complexity to the theoretical treatment: the presence of the electrolyte at the electrode surface and varying electrode potentials. Still, there are promising approaches to cope with these problems allowing a realistic theoretical description of reactions in electrocatalysis. It will be demonstrated that ab initio molecular dynamics simulations based on periodic density functional theory calculations can contribute to an understanding of the structures and reactions at water/metal interfaces. In order to model varying electrode potentials, an explicit counter electrode has been implemented in a periodic density functional theory code, and first preliminary results using this implementation will be presented. (c) 2010 Elsevier B.V. All rights reserved.