This paper describes an interactive computer program that can be used as a design tool in assessing the shading effects of trees in relation to buildings. The program determines the area and position of the shadow cast by a tree or group of trees on a surface of a building which can have any orientation. The program outputs numerical and · graphical hourly results at any time of the day, for any day of the year, at any location on the earth, for any relative positioning between the tree and building, and estimates percent irradiation reductions on building surfaces resulting from tree shade. The program allows three different shapes of tree (elliptical, cylindrical, and parabolic) to be used. This program is being interfaced with a commercially available building energy analysis program.