Medicinal chemists have tried to quantify relationships between chemical structure and biological activity since before the turn of the century. However, it was not until the early 1960s, through the efforts of Corwin Hansch and his co-workers, that a workable methodology was developed and the subject that was to become known as quantitative structure-activity relationships (QSAR) was born. Since then, thousands of research papers, articles and reviews on QSAR have emerged, with unfamiliar symbols and parameters, and with results which are expressed in a format different from that of traditional medicinal chemistry. It is the object of this chapter to explain these methods of expression, what they are meant to convey, and how the technique may be used in drug design.