Status of polarized lepton-ion colliders

Lepton-Ion colliders may provide a decisive tool to explore so far unresolved problems in fundamental research, in particular those concerning the QCD-structure of the nucleon. Collaborations in the United States (ELIC and eRHIC) and in Europe (ENC-FAIR and LHeC) pursue different collider designs. In most cases, both colliding species will be spin polarized. Concentrating on the accelerator physics issues this paper gives an overview of specific design features and challenges.