Sulfonation is an important conjugation reaction required for a range of biological processes including phase II metabolism, whereby sulfo-conjugation renders a compound more hydrophilic to aid its excretion. The major enzyme responsible for xenobiotic sulfonation is the widely expressed cytosolic sulfotransferase SULT1A1. The SULT1A1 crystal structure has provided insights into this enzyme's substrate specificity and catalytic function, including its role in the sulfonation of endogenous substrates such as oestrogens. Contrary to its metabolic role, SULT1A1 can also bioactivate compounds; it is known to sulfonate pro-carcinogens such as hydroxymethyl polycyclic aromatic hydrocarbons leading to highly reactive intermediates capable of forming DNA adducts, potentially resulting in mutagenesis. Given the role of SULT1A1 in these diverse functions and the discovery of allelic variants with differing catalytic activities, this enzyme has been the focus of numerous polymorphic studies investigating the link between inter-individual SULT1A1 variance and the etiology of a variety of cancers.