Our favorite tabletop role-playing games (TTRPGs) provide a chance to act out a fantasy. It might be the desire to wipe out foes with a single strike, the ability to charm even the most curmudgeonly townsfolk, or maybe even just the aspiration to adventure as a potted plant. Since game systems don’t always let players craft the kinds of games they want to experience, this is when they turn to the “homebrew” process to create personal rules and worlds. While Pathfinder has a history of being an underdog to the longtime industry leader Dungeons & Dragons, it has an ethos of open game design that inspires players, indie makers, and third-party content creators to brew up new rules, characters, classes, and game settings. Their System, Our Game looks at the history of Pathfinder and how it encourages aspiring makers to break into the game design space. Through interviews with player-creators and third party makers, the authors use a range of perspectives to define homebrew for TTRPGs. Taking a closer look at homebrew design, they create two homebrew campaigns (available as free downloads) collaboratively with two different groups of players. Player accounts of gameplay and designer reflections of how their homebrew actually played out at the game table outline some of the challenges and perks of designing on the fly, creating customized settings, and bending or breaking rules. The book concludes by going through the research, interviews, design, and play analysis to show what homebrew and Pathfinder mean together. Ultimately Their System, Our Game is a celebration of how players use homebrew to explore creation and expression in their own TTRPGs.