Electrocatalytic studies on bimetallic single crystal electrodes present a unique opportunity to explore the reactivity of complex surfaces with known structure and composition. Such electrochemical studies, together with measurements in ultra-high vacuum, provide the theoretical and experimental basis for rational design of more active electrocatalysts. Pure platinum, though the most active single-component electrocatalyst for many reactions, is still not active enough for some applications, particularly in fuel cell technology, and cannot be considered as a true catalyst for most electrocatalytic processes. Therefore, a concerted effort has been made in the last 40 years to enhance the electrocatalytic activity of Pt via modification by a second metal. Most early work used polycrystalline alloys, but in recent years, many workers have begun modifying (decorating) Pt single crystals by deposition of a second noble metal, which is usually ruthenium, palladium, rhodium, osmium, or silver. A critical review of electrocatalysis on such well-defined bimetallic surfaces is offered, and a brief analysis of the inverted approach, in which single crystal noble metal surfaces are modified by Pt deposition, is also presented.