Inspired by the notions of shape grammars, a generative design grammar is a set of rules that describe a design style. This chapter describes and demonstrates how such a grammar can be developed to design places in 3D virtual worlds, using the scenario of a virtual gallery. By encoding the design style as a grammar, the rules can be executed to generate many different individual designs sharing the style, in contrast to the current practice of handcrafting each individual design. The intelligent design of places in 3D virtual worlds is then realized through a Generative Design Agent (GDA) by at first applying the grammar to generate the initial design, and then adapting the design during real-time interactions in virtual worlds. As a result, this design grammar is integral to the 3D model, providing the algorithmic process through GDAs to modify the virtual world in response to the needs of its inhabitants in real time. This represents a new role for design grammars in which the grammar enables real-time design and modification of a virtual world.