Although early attempts to use metal alloys to probe the so-called electronic factor in catalysis by metals did not meet expectations, investigations of bimetallic catalysts with other objectives have led to major advances. Such investigations have shown that bimetallic catalysts can exhibit selectivities for hydrocarbon transformations that are very different from those observed on monometallic catalysts, favoring the formation of valuable products such as aromatic hydrocarbons at the expense of less valuable ones such as methane. The number of contiguous atoms of a given metal component required in an active catalytic site is an important issue here. Investigations with bimetallic catalysts have also shown that systems of interest are not limited to combinations of metallic elements that are highly miscible in the bulk. For industrial applications of bimetallic catalysts, a crucial advance was the demonstration that highly dispersed bimetallic entities as small as 1 nm in size could be prepared within the pores of materials commonly used as catalyst supports. The term “bimetallic clusters” is used in referring to these entities. A very important industrial application of bimetallic catalysts has occurred on a huge worldwide scale in petroleum reforming processes. This application has played a vital role in making lead-free gasoline a reality, an accomplishment of enormous environmental significance.