Protein tyrosine O-sulfation is a posttranslational modification that takes place in the trans-Golgi network and occurs for a wide spectrum of transmembrane and secretory proteins found in all animals and in the plant kingdom. The sulfate transfer reaction to tyrosine residues is catalyzed by one of two tyrosylprotein sulfotransferases (EC 2.8.2.20) which use the universal sulfate donor 3′-phosphoadenosine 5′-phosphosulfate. Protein tyrosine O-sulfation promotes extracellular protein–protein interactions occurring in various biological processes, ranging from receptor binding of regulated peptides to the interaction of viral envelope proteins with cell surface proteins. An overview of our knowledge of protein tyrosine sulfation is presented.