The Euphorbiaceae is a large family of dicot angiosperms consisting of 326 genera and 7750 species (Mabberley, 1987). The family is nearly cosmopolitan in distribution and most are shrubs or trees, many xerophytic in habit. Leaves of more than 1700 species were collected from the Rijksherbarium at Leiden and the Missouri Botanical Garden Herbarium at St. Louis. These leaves were cleared and photographed and arranged for description according to the classification proposed by Webster (1975). In this classification the family is divided into 5 subfamilies and 52 tribes. Venation patterns of slightly more than 700 species from the first two subfamilies, consisting of 17 tribes, will be described in this volume.