An overview about the effect of co-metal on the performance of Pd in bimetallic catalysts is presented. In the first part, the promotion of monometallic catalysts by addition of a second metal is described from a general point of view in terms of: (i) electronic effects; (ii) geometric effects; (iii) the occurrence of mixed sites; and (iv) the disappearance of the beta -PdH phase. The second part presents the main methods to prepare bimetallic catalysts through the unselective and selective depositions of the co-metal precursor and the use of heterobinuclear complexes. The influence of preparation methods on the chemical state and the spatial distribution of both components are highlighted. In the last part, a short overview is given about the influence of co-metal addition on some important reactions in which Pd is the reference active phase: (i) selective hydrogenation of highly unsaturated aliphatic hydrocarbons; (ii) hydrogenation of aromatics or hydrodearomatisation; (iii) hydrogenation of nitrogen-containing compounds, (iv) reactions involving CO; and (v) hydrodechlorination.