Human-robot interaction (HRI) takes place especially through interfaces. The design of such interfaces is a very delicate and crucial phase because it influences the robot accessibility and usability by the user. In this paper, we describe and analyze the results of 2 tests conducted so as to understand some of the optimal features that should characterize the robot voice and graphical-based user interfaces. Our test platform is an assistive robot developed for the elderly with mild cognitive impairments. Therefore, the user interfaces must be clear and simple. The ambiguities must be eliminated so as to facilitate the use of the robot and hence not to discourage the elderly population to use new technologies.