Ornithine lipids (OLs) are phosphorus-free membrane lipids relatively common in eubacteria, but absent from archaea and eukaryotes. In addition to ornithine, other amino acids (and even di- and tripeptides) such as glycine, glutamine, lysine, serine-glycine, and ornithine-serine-glycine have been described as headgroups of these lipids in some bacterial species. Structurally, they are composed of a 3-hydroxy fatty acid amide bound to the α-amino group of ornithine or another amino acid and of a second fatty acyl group ester linked to the 3-hydroxy position of the first fatty acid, forming an acyloxyacyl structure. In the case of OLs, this basic structure can be modified by hydroxylation at different positions, by N-methylation, or by taurine transfer. The presence of OL and/or modified OLs often seems to form part of a stress response to (changing) environmental conditions. OL modifications enable bacteria to adjust membrane properties by converting existing membrane lipids into lipids with distinct properties, without requiring de novo synthesis. In recent years, enzymes involved in the synthesis of glycine lipids and glutamine lipids have also been identified and characterized.