Business units in six Fortune 500 companies were studied to develop better understanding regarding drivers of product portfolio complexity and the means to manage them. Our research focuses on identifying important competencies for managing product portfolio complexity and on the development of appropriate theoretical explanations. We found three important competencies: product/technology portfolio strategy, organization and governance regarding complexity decisions, and product design and decision support systems. We explicate these competencies using a socio-technical systems theoretical perspective. Our findings provide the basis for a model describing the impact of complexity and complexity management on business unit profitability.