Addition reactions are reactions in which two or more molecules combine to form a larger molecule, with no other products. No atoms or molecules get eliminated in this reaction. An example of addition reaction is the hydrogenation of alkenes to form alkanes.

Unsaturated compounds i.e., compounds containing double or triple bond(s) undergo addition reactions very easily. To accommodate the new incoming atom(s) (in the case of hydrogenation, the incoming atom is hydrogen), the double/triple bond(s) gets cleaved and converted into single bonds such that all elements present in the compound get a stable electronic configuration.