Cysteine dioxygenase (CDO) catalyzes the oxidation of cysteine to cysteine sulfinic acid, which is the first major step in cysteine catabolism in mammalian tissues. Crystal structures of mouse, rat, human and bacterial CDO have recently become available and provide significant mechanistic insights. Unlike most non-heme Fe(II) dioxygenases, coordination of the Fe in CDO deviates from the 2-His-1-carboxylate facial triad archetype and instead adopts a His3 facial triad. This change is expected to have an influence on oxygen activation by the catalytic site. The structures also reveal the presence of a cysteinyltyrosine (Tyr157-Cys93) post-translational modification near the active site. Kinetic studies of mutant CDOs reveal that the cysteine residue is less critical than the tyrosine for enzyme activity. Inconsistencies about the details of the active site and the nature of substrate binding exist and are discussed. Herein we review the structural biology along with relevant kinetics studies that have been conducted on CDO for insights into the reaction mechanism of this novel non-heme iron dioxygenase.