Tyrosine (Tyr) sulfation is a common posttranslational modification of secreted proteins or membrane-bound proteins that is implicated in numerous physiological and pathological processes. The Tyr sulfation modifies proteinprotein interactions involved in leukocyte adhesion, homeostasis, and receptor-mediated signaling. To data, 80 Tyrsulfated proteins have been identified. As new methodologies and bioinformatics for the detection of Tyr sulfation become available, the number of Tyr-sulfated acceptor proteins discovered is bound to increase. Further, recent advances in microscopy and fluorescence technology will provide information on the true spatial and temporal nature of Tyr-sulfated proteins within the intact cell. This review summarizes the methods for the detection of Tyr O-sulfation as well as the biological functions of sulfated Tyr. Further, illustrative examples of the impact of Tyr sulfation on the pharmacological properties are presented.