This thesis focuses on a set of digital musical instruments I have designed and developed with ensemble improvisation in mind. The intention is not to create a universal improvisational instrument, but rather to create a set of instruments which each realize one musical idea. My research addresses the meaning of relations between activities in two stages, what I call "design time" and "play time". In short, Design time is conception, representation and articulation of ideas and knowledge outside of chronological time, whereas play time takes place in real-time and concerns bodily activity, interaction, and embodied knowledge. In this work aesthetics play a crucial role, and here signify what is important for me. At design time my aesthetic preferences guide the designprocess, whereas in play time, a subjective aesthetic tenet is that musical improvisation has strong similarities to gaming and play. One hypothesis states that choices made during the design process at the development stage of a digital musical instrument significantly influence ensemble improvisation and musical results at play time. A digital instrument in this work constitutes a field of possibilities, which in play actualizes the aesthetic decisions of its designer, and cases where the designer and player are one, during play there will be a double influence: directly through the player's actions, and indirectly through the nature of the instrument.