The multiple physiological functions of steroid hormones have been known for many years. The cloning of the steroid receptors in the mid-1980s led to the concept of ligand-activated transcription factors and to the identification of specific DNA response elements in the regulatory regions of target genes. The next main development was the identification of cofactors with activating or repressing functions, of which several act by modifying histones and locally affecting the chromatin structure. Work from several groups shows that the steroid receptors themselves can also be modified at various positions. Besides the long-known phosphorylation at tyrosines and serine/threonine residues, other covalent additions such as acetylation, ubiquitylation and sumoylation have been evidenced for steroid receptors in recent years. These modifications affect receptor stability and activity, and provide potential mechanisms for cell- or gene-specific regulation. A better understanding of the impact of these post-translational modifications (PTMs) on steroid receptor function should help in the identification of novel ligands with improved clinical profiles.