the organizational field of open source software, participants perceived software as a technical device, and they looked upon themselves as programmers and users modifying and sharing codes, making them available to everyone for free. Within the field of commercial companies, managers and employees perceived software as a commodity that could be bought and sold, and the development of the software was shrouded in secrecy and wrapped in copyrights and licenses. By the end of the millennium, people from the two organizational fields began to interact with each other – and today, commercial companies are involved in activities within open source software communities in different ways.  ‘How did people start to co-operate with the ‘enemy’ on software development’ is the leading question in the book. The answers are based on in-depth studies of three empirical cases showing different variations of co-operations and different ways and degrees of successful co-operation. In all three cases the development has raised serious identity questions like: Who am I? Who are my friends and enemies? And what is the right thing for me to do in the future? The answers have not been found by just following a logic of consequentiality, where people know who they are and how to pursue their interests. Answering these questions, by contrast, has been a highly ambiguous process for many people within both fields. The process has been filled with uncertainty and emotions, and it has in varying degrees involved institutional work and interactions between many and different types of institutional actors: institutional entrepreneurs, institutional audiences and institutional leaders.  The book is for everyone interested in software development and/or open innovation processes and will be of particular interest for organizational scholars as it draws heavily on sociological concepts like institutional logics, institutional work and institutional actors.