Apart from their practical value as a source of food to our wildlife, acorns are of interest to the student of nature by serving to illustrate variation in fruit and as a means of identifying the different species of oak trees. To most of us an acorn is simply an acorn, and seemingly one acorn is much like another. Actually, these nuts are quite variable in shape and size, in the appearance of the cup, and in other features. These various characters have been used in setting up the following key for the convenience of those wvho may desire to identify the different species of oaks. The oaks are divided into two groups: the white oak group and the black oak group. There are a number of structural differences that differentiate the members of these two groups, but the one with which we may be concerned is the fact that the acorns of the black oak group take two years to mature. Hence, any acorn found on the twigs of an oak during the winter or spring must obviously belong to the black oak group. However, it must be borne in mind that since the nuts are not mature the description of the nuts is not fully applicable and the determination must therefore be made essentially on cup characters.