Experimental prototyping and play testing using iterative game design is discussed. Prototypes allow designers to base decisions on successive iterations of a game as it is tested and refined. Different types of prototypes are described from early paper prototypes to later software prototypes. Various types of play tests are also outlined such as focus tests, expert evaluations, and free-form play sessions. Obtaining useful data from play tests to answer specific research questions is the focus. Methods like surveys, interviews, video recordings, and data mining game interactions are presented.