Network slicing is a technology that will change the way in which future networks and services will be designed and operated. The approach impacts telco operators from the technical and business point of view. The technology is a subject of intensive research and standardization. The on-going work on network slicing has shown that many issues still have to be solved before the solution can be used by telco operators. Currently, the technology does not fulfill telco functional and performance-related requirements. In this paper, we describe the present status of network slicing and list the network slicing-related issues that still have to be solved. The list includes issues related to the scalability of slice management and orchestration, end-to-end slice orchestration, data plane and RAN slicing problems as well as business-oriented and legal issues.