Abstract: TreeSketch is a system for modeling complex trees that look natural yet are creatively designed. The system integrates procedural tree generation with a multi-touch tablet interface that provides detailed control of tree form. The procedural component is based on the concept of tree self-organization and simulates competition of branches for space and light as the tree develops from a seedling. The modeler can control this process by directing growth with a procedural brush, changing parameters as the tree grows, interleaving controlled and autonomous growth, and editing generated forms. Complex trees can be created in a matter of seconds.