The human catechol-O-methyl transferase (COMT) gene contains multiple single-nucleotide polymorphisms, some of which are postulated to have clinical significance. This article reviews human studies that have explored the association between COMT polymorphisms and addiction to drugs, alcohol or tobacco. Most studies concentrate on the Val108/158Met polymorphism. Although there are reports indicating a positive association with COMT polymorphisms and addiction, the majority of the studies failed to detect such a link between them with one exception, smoking. It is unlikely that there would be any single gene that could be designated as 'the addiction gene'. Rather, there seems to be a great number of genes that are associated with addiction, among which COMT seems to have a minor role. Environmental factors and genetic milieu have a great impact on whether the small effects of COMT polymorphisms on risk of addiction can be detected in a given population. Sex differences complicate the gene-environment interplay even further.