Many nuclear, mitochondrial, and cytoplasmic proteins are modified by monosaccharides of O-linked β-N-acetylglucosamine (O-GlcNAc). In mammals, O-GlcNAc is dynamically added and removed from proteins by just two enzymes, the O-GlcNAc transferase (OGT) and the O-GlcNAcase (OGA). On a subset of proteins, O-GlcNAc competes with phosphorylation for Ser/Thr residues. Evidence suggests that some proteins are rapidly cycled between phosphorylated and glycosylated states by protein complexes composed of OGT and protein phosphatase 1 β, γ, or OGA and protein kinases.