Leonardo da Vinci's drawings of 'grotesque' heads have fascinated writers and art historians from Giorgio Vasari (1511-1574) onwards. These drawings enjoyed a considerable popularity among Renaissance artists and copies of them were made well into the nineteenth century, both in Italy and the North. Though various theories have been advanced to explain the meaning of these drawings, they have never been the subject of a monograph. The present study seeks to explain Leonardo's lifelong habit of drawing striking and monstrous heads by relating it to his writings on painting and the human figure and the current science of physiognomy. Taking into account the distinctive appearance of each of Leonardo's figures and the implications of his views and those of his early commentators, this book offers for the first time a classification of this special group of drawings into categories of purpose.