This thesis sets out to explore why people engage in, and how to design for, play in a public setting. It does this by separating design for play from design of games, describing play as a socially and mentally understood activity, and a playful approach to engaging in that activity. It emphasises that while play is voluntary, design can help shape the players’ mode of engagement. The thesis uses a qualitative and inductive approach to research, with an understanding of knowledge as being constructed in the individual. The research is grounded in human computer interaction and interaction design, and closely related to game studies and design science. The research question concerns how design can influence the player activity in order to create a desired player experience in public, by harnessing playful engagement. It’s foundation is a theory of play which describes play as a framed, or hedged-off, activity with a fragile border; where knowledge and feelings can leak both in and out of the activity, and affect the play as well as what is around it. The theory of enjoyment of play is discussed, and the problem of treating this as ‘fun’ is addressed, concluding in a presentation of how playful engagement can be harnessed through design. The theory is applied in five design cases: I’m Your Body, a locative storytelling app; Codename Heroes, a pervasive game of personal empowerment; Passing On, a slow-paced game about communication; Busking Studies, which involves observing street performers and their shows; and DigiFys, an architectural design exploration of playgrounds and play paths. Finally, three concepts, or design tools, are presented, which address: 1) a structure for understanding a design through three layers, constructs designed by the designer, inspiring play activity with the player, leading to experience; 2) an approach to designing invitations to play; and finally 3), a four faceted structure for understanding play engagement when players engage in non intended ways.