The methods of molecular quantum mechanics have been used to study the important biological molecules glutathione, ascorbic acid and related compounds. The preferred conformations of glutathione, its constituent amino acids and their residues have been investigated using the semi-empirical PCILO procedure. Comparisons with ab initio calculations are given for the smaller molecules. The electronic structure and properties of ascorbic acid and its metabolites have been studied using ab initio procedures. The calculations include geometry optimizations, electrostatic molecular potential maps, spin density calculations on the radical species and investigations of charge transfer interactions and of metal complexing.