This research provides guidelines for the representational and interaction design of virtual invehicle assistants. The guidelines offer a clearly structured overview about what designers have to consider while creating in-vehicle virtual assistants with a convincing user experience. The design guidelines are developed using the design science research methodology. Based on assistant requirements for a great user experience derived from the previous research literature and the results of five expert interviews, concrete guidelines for the design of invehicle virtual assistants are developed. For evaluation, the guidelines are presented to another expert and then refined based on her feedback.