In this chapter, we will present a theory of play dynamics. The effect of play dynamics is to push users into a state of play, which is a special way of being in the world, with a certain framing and disposition towards the world and the actions performed. We believe that humans play solely for fun, and that they seek tools that use play dynamics to push them into a state of play, creating feelings of joy. In this chapter, we will present examples of play dynamics in well-known play tools, from physical movement to computer games. We believe that the concept of Play Dynamics facilitates deeper understanding of why certain play tools work better than others and why.