Games are shaped by the tools we use to make them and our ability to model the concepts they address. Vast improvements in computer graphics technology, processing power, storage capacity, and physics simulations have driven game design for the past forty years, leading to beautiful, spacious, detailed, and highly immersive worlds supporting games that are, for the most part, fundamentally about movement, collision, and other physics-based concepts. Designers use increasingly complex tools that support the creation of these worlds. The games industry is undoubtedly thriving on these technologies, but what comes next? The tools that game designers use today are highly complex--too complex for a growing population of novice designers, and offering little support for reasoning about gameplay--and there are entire genres of games that cannot be made using the tools available to us now.

This dissertation presents the use of procedural content generation to create expressive design tools: content generators that are accessible to designers, supporting the creation of new kinds of design tools and enabling the exploration of a new genre of game involving the deep integration of procedural content generation into game mechanics and aesthetics. The first of these tools is Tanagra, the first ever AI-assisted level design tool that supports a designer creating levels for 2D platforming games. Tanagra guarantees that levels created in the tool are playable, and provides the designer with the ability to modify generated levels and directly control level pacing. The second tool is Launchpad, which supports a designer controlling both component and pacing features of generated levels; its companion game Endless Web uses the generator to create an infinite world for players to explore and alter through their choices. Endless Web is one of a handful of games in a new genre enabled by content generation: PCG-based games. Finally, this dissertation presents a novel method for understanding, visualizing, and comparing a generator's expressive range, thus allowing designers to understand the implications of decisions they will make during the design process.