This paper investigates forms and functions of user feedback in a map task dialogue between a human and a robot, where the robot is the instruction-giver and the human is the instruction- follower. First, we investigate how user acknowledgements in task-oriented dialogue signal whether an activity is about to be initiated or has been completed. The parameters analysed include the users' lexical and prosodic realisation as well as gaze direction and response timing. Second, we investigate the relation between these parameters and the perception of uncertainty.