Bimetallic catalysts play an important role in many industrial catalytic processes and represent an area of intense research interest. Bimetallic systems give an opportunity to investigate geometrical versus electronic or ligand effects in catalysis. Studies of these effects can help to elucidate the key factors controlling the activity and more importantly, the selectivity of metal catalysts. For the designer of catalysts, adding a second metal component allows to systematically alter the size or electronic structure of catalytic surface ensembles. One could envision modifications of the adsorption characteristics, of the surface coverage with reactive intermediates, and consequently of the selectivity. The presence of a second metal component can make its influence felt in terms of changes in reducibility of the catalyst. Furthermore, the second metal component can in certain cases alter the deactivation behavior, a fact that has proven to be beneficial in bimetallic reforming catalysts. This chapter focuses on the modification of surface sites by addition of a second metal component and on the direct conversion of synthesis gas over organometallic clusters or cluster-derived catalysts that are not entirely subject to the Anderson-Schultz-Flory polymerization kinetics. The chapter reviews the methanation process, Fischer-Tropsch catalysts modified by addition of a second metal component, and bimetallic catalysts derived from organometallic clusters. It seeks to present an overview of supported bimetallic CO activation catalysts and to highlight major trends and fundamental principles.