In this chapter, we discuss the design of tangible interaction techniques for Mixed Reality environments. We begin by recalling some conceptual models of tangible interaction. Then, we propose an engineering-oriented software/hardware co-design process, based on our experience in developing tangible user interfaces. We present three different tangible user interfaces for real-world applications, and analyse the feedback from the user studies that we conducted. In summary, we conclude that, since tangible user interfaces are part of the real world and provide a seamless interaction with virtual words, they are well-adapted to mix together reality and virtuality. Hence, tangible interaction optimizes a users' virtual tasks, especially in manipulating and controlling 3D digital data in 3D space.