Everyone speaks of it, everyone believes to know about it. But what are the theories behind outsourcing? What are decisions based on? Is it just a cost-benefit decision? Outsourcing is a very complex phenomenon, and with the rapidly changing business environment, it calls for further clarification and research. This work considers various theories related to outsourcing, business environmental factors, and regards the work of Michael E.Porter on competitiveness and core competences. It analyses the phases of outsourcing and provides a five step decision-making framework.