Free Libre Open Source Software (FLOSS) is of vital importance for the daily life of many private and corporate users. However, the majority of all FLOSS initiatives fail, most commonly due to a lack of sustained developers. In contrast to previous research which used an individual centric or a structural perspective, this dissertation combines motivational and relational aspects to build a comprehensive understanding for FLOSS developers' ongoing project commitment. A unified research model is developed by drawing on established theories from organizational and sociological literature, in particular by combining Self-Determination-Theory (SDT) and Social-Identity-Theory (SIT). Both SDT and SIT have been found valuable concepts for staffing decisions in organizations. In addition to the development and evaluation of the research model, this dissertation derives operational strategies for project managers of FLOSS initiatives on how to enhance the retention behavior of their contributor base.