Abstract: Oxonium ions, molecules with a trivalent oxygen atom that carries a formal positive charge, have a rich history in chemistry. The simplest and most prevalent is the hydronium ion – H3O+. Trialkyl oxonium ions, e.g. Meerwein’s salts (Me₃O+ BF₄– and Et₃O+ BF₄–) are less common and are some of the most potent alkylating agents encountered in the laboratory. Although the hydronium ion is ubiquitous in chemistry and biology, trialkyl oxonium ions are rarities in both areas. Interestingly, a number of halogenated natural products isolated from Laurencia species, are proposed to arise biosynthetically via complex trialkyl oxonium ion intermediates. Good circumstantial evidence for the intermediacy of such oxonium ions has been provided through biomimetic total syntheses of a number of Laurencia natural products. This lecture will describe our work on the synthesis and characterization of four of these biosynthetically relevant oxonium ions  which represent the most structurally and stereochemically complex oxonium ions characterized to date. The utility of these oxonium ions in the total synthesis of more than ten halogenated natural products from Laurencia species will also be described.