In the management of national electronic identity (eID) infrastructure, cooperation between public and private parties becomes more and more important, as the mutual dependencies between the provision of e-services and the provision of the national public key infrastructure (PKI) continuously increases. Yet, it is not clear which key factors affect the public-private collaboration in the eID field, as existing studies do not provide insight into this particular matter. Therefore, we aim to identify the factors that affect public-private partnership (PPP) in the field of eID. We also describe feasible formats that help to improve the cooperation between the two sectors, based on insights from the case of Estonia. In service of that study, we conducted twelve qualitative interviews with high-level experts representing several parties from the public and the private sector. By conducting a thematic analysis of the interviews, we identified five key factors for successful PPP in the eID field, i.e., engagement, joint understanding, two-way communication, clear role division, and process orientation. Furthermore, we generalize our results by discussing, in how far the found cooperation formats can be used by stakeholders to manage state-critical information technology (IT) infrastructure components similar to eID such as mobile phone services, data transmission services and digital signature services.@en