Sialic acids are structurally diverse molecules that have important roles in the physiological reactions and characteristics of prokaryotes and eukaryotes. These include the ability to mask epitopes on underlying glycan chains and to repulse negatively charged moieties. Here, we describe the metabolism and immunological relevance of sialic acids and outline how their properties have been exploited by the pharmaceutical industry to enhance the therapeutic properties of proteins such as asparaginase and darbepoetin alpha.