The source characteristics of the European XFEL and the planned experimental facilities that are relevant for the X-ray detectors are presented, and the requirements for the 2-dimensional X-ray Detectors are stated and explained. It is clear that, although these requirements will evolve with time, they demand new detector concepts to be developed. Three X-ray detector development projects have been initiated by the European XFEL, each using a conceptually different approach to meet the stringent requirements. The basic principles used in the projects are briefly presented.