The aim of this doctoral project has been to study so-called non-idiomatic improvisation in ensembles consisting of two or three musicians who play together without any restrictions regarding style or genre and without having predetermined what is to be played or how they should play. The background to this thesis has been the author's own free improvising, which he has pursued since 1974, and the questions that have arisen whilst music-making. The thesis takes three of these questions as its point of departure: - what is free ensemble improvisation, what characterizes free ensemble improvisation and how can it be defined - how does free ensemble improvisation relate to: - - instrumental technique - - idiomatic improvisation and stylistic influences - - composition - -interpretation - - aleatorics and indeterminacy - - different types of sytems (e.g. biological, social, dynamic/chaotic systems) - what might a conceptual model as a theoretical base for free ensemble improvisation look like? The artistic/performative part of this research project has primarily consisted of public concerts, as a result of longer/shorter periods of cooperation with four permanent and a number of temporary (ad hoc) ensembles. The results provide a better understanding of what free ensemble improvisation is, in what respects it differs from other forms of music-making and how it can be defined. Free ensemble improvisation's relations to the points mentioned above were found to be more multifaceted than expected. However, it was possible to attain a basic twolayered conceptual model as a theoretical base for free ensemble improvisation and, in its extension, as a basis for the analysis of free ensemble improvisation. The study includes numerous concert projects, of which several are recorded and included in this book on two CDs with MP3 files.