The constant interaction of users with different Digital Libraries (DLs) and the subsequent scattering of user information across them raise the need not only for Digital Library interoperability but also for cross-Digital Library personalization. The latter calls for sharing and combining of user-information across different DL systems so that a DL system may take advantage of data from others. To achieve this goal, DL systems should be able to maintain compliant and interoperable user models and profiles that enable propagation and reconciliation of user information across different DLs. In this paper, we motivate the need for cross-Digital Library personalization, we define and examine user model, profile, and context interoperability, and we survey and discuss existing user model interoperability approaches.