Procedural representations provide powerful means for generating complex geometric structures. They are also notoriously difficult to control. In this paper, we present an algorithm for controlling grammar-based procedural models. Given a grammar and a high-level specification of the desired production, the algorithm computes a production from the grammar that con-forms to the specification. This production is generated by optimizing over the space of possible productions from the grammar. The algorithm sup-ports specifications of many forms, including geometric shapes and analytical objectives. We demonstrate the algorithm on procedural models of trees,cities, buildings, and Mondrian paintings.