Fic enzymes post-translationally modify proteins through AMPylation, UMPylation, phosphorylation, or phosphocholination. They have been identified across all domains of life, and they target a myriad of proteins such as eukaryotic GTPases, unstructured protein segments, and bacterial enzymes. Consequently, they play crucial roles in eukaryotic signal transduction, drug tolerance, bacterial pathogenicity, and the bacterial stress response. Structurally, they consist of an all α-helical core domain that supports and scaffolds a structurally conserved active-site loop, which catalyses the transfer of various parts of a nucleotide cofactor to proteins. Despite their diverse substrates and targets, they retain a conserved active site and reaction chemistry. This catalytic variety came to light only recently with the crystal structures of different Fic enzymes.