Spring frosts add considerably to the difficulties of establishing young plantations. Investigations which the Imperial Forestry Institute had been carrying out for the Forestry Commission since 1929 were already yielding interesting results when the great May frosts of 1935 occurred. These frosts were so widespread and did so much damage that it was decided to study the whole subject in detail. This Bulletin deals with the problems of how to recognise frosty areas in advance of planting, and how to assess the frost-hardiness of different species of trees. Detailed attention is given to the climate of the ground air zone within which trees live and to susceptibility of forest trees to damage by spring frost. The main concern is with trees of economic importance but the lists indicating the relative frost-hardiness of numerous ornamental trees and shrubs will perhaps also be useful to gardeners and others who plant for amenity.