Currently, international research on the nonprofit sector is paying increasing attention to institutional as well as organizational analysis. Drawing on both approaches, this article seeks an understanding of the changing configuration of the sector by investigating the dimension of flexibility in the relation between nonprofits and their various dynamic environments. The approach is developed for the German case. The analysis not only permits identification of the underpinnings of the old "corporatist model," shaping the German third sector as a social sphere between the state and the market economy, but also points at the factors leading to a gradual erosion of that model. Moreover, it allows a brief exploration of the evolutionary dynamics of the German nonprofit fields in comparison to major international trends.