The aim of this chapter is to introduce the basic challenges and tools involved in modeling chemistry on metallic surfaces, and to provide practical guidance in the use of these methods and tools to design quantitative simulations of surface processes. Calculations involving a metal surface introduce many additional complications relative to quantum chemistry studies of molecular chemistry. These not only include new methods and unfamiliar tools but also novel issues and unexpected pitfalls. Thanks to continuing advances in methods and computational capabilities, –rst principles simulations of surface chemistry are becoming generally accessible. These advances are founded upon the success of density functional theory (DFT) and the development of large-scale solid-state codes implementing DFT for periodic systems. The size and complexity of –rst principles simulations has become staggeringly large compared to what was feasible decades ago, yet the codes can be deceptively simple to use. The advanced capabilities of modern computational tools rely on many layers of complexity with many hidden options and assumptions, and it is possible that concealed approximations may ultimately limit the accuracy of a simulation. Obtaining quantitatively meaningful calculations for surface chemistry requires awareness of the physical approximations inherent in the methods and careful design of a viable model. Improper use of these tools can result in drastically unrealistic conclusions. Some general considerations along these lines for solid-state calculations in materials science were considered in a recent review article . Here, we describe several fundamental issues that arise in applying DFT methods to surface chemistry, calculations that explore the intersection between physics and chemistry, the realm of metallic clusters and surface catalysis.