Most drugs are weak acids or bases and the acidity or basicity (as measured by the acid-base dissociation constant pKa) may influence not only the pharmacological profile but also the physicochemical properties of the molecule. The latter have a profound effect on absorption, distribution, metabolism and excretion (ADME properties). This lecture will focus on the effect of structure on pKa and how to calculate the percent ionisation of an acid or base at different pH.