Achieving higher activity and selectivity of heterogeneous catalysts, electrocatalysts, and sensors requires advances in controlling structure and chemistry relevant to interfacial reactions at the nanoscale. One can now exploit an unprecedented ability to investigate such phenomena on alloy surfaces to obtain new information about how and why composition, structure, and defects alter chemical reactions that occur at specific sites. We have been probing this site-directed chemistry at alloy surfaces in a wide range of chemisorption and catalytic reactivity studies. The talk today will focus on how recent results for several Sn/Pt(111) and Sn/Pt(100) ordered surface alloys have helped to define the overall chemical reactivity of Pt-Sn bimetallic surfaces, clarified the role of a second metal in altering the chemistry of Pt alloys, and led to general principles for understanding the reactivity and selectivity of alloy catalysts. Specifically, I will discuss exploiting Pt-Sn alloys for selective hydrogenation of 1,3-butadiene based on studies of well-defined model catalysts, i.e., Pt(111) and the (2x2)-Sn/Pt(111) and (@sr@3x@sr@3)R30°-Sn/Pt(111) surface alloys, that probe the influence of alloyed Sn on the reaction barrier to butadiene hydrogenation and the effect of surface Sn concentration on hydrogenation activity and selectivity. Fundamental concepts emerging from such studies enhance our understanding and ability to tailor local properties of alloy surfaces, which should facilitate the design of new catalysts and sensors.