Application developers are of growing importance to ensure that smartphone software platforms gain or maintain a competitive edge. However, despite the increasing research interest in platform openness, previous research attempts have neglected to investigate the perspective of third-party developers. This paper contributes to a more sophisticated understanding of the third-party developers' individual perception of platform openness by identifying concrete facets of openness and evaluating their impact on the developers' work. For this purpose, a comprehensive qualitative investigation and a quantitative survey were undertaken. Based on our findings, we discuss practical implications regarding the management of third-party developers in smartphone platform ecosystems.