The goal of this thesis has been to introduce the concept of playful interaction as an alternative or extension to traditional efficiency focused interaction design. Playful interaction requires effort from the user and rewards her with enjoyable skill improvement. Furthermore, playful interactions are strong sources of emotion. The concept is further explained as consisting of at least three categories - playful selection, playful activation and playful guidance - each playful in their own way and for their own reason. Playful selection will enrich a user interface with game-like mechanics with either learning before time or simple enjoyment as the reason. Playful activation replaces trivial interactions with small challenges to increase user attention and interest. Playful guidance provides integrated, just-in-time learning and help and hints as satisfying rewards or achievements. To demonstrate the concept it is added to a game prototype called Project Sprout, which is then evaluated with and without playful interaction. The evaluation shows that parts of the playful interaction concept does indeed work, but fails to provide enough data to cover all the categories of playful interaction.