The paper describes a multi-modal scheme for human-robot interaction suited for a wide range of intelligent service robot applications. Operating in un-engineered, cluttered, and crowded environments, such robots have to be able to actively contact potential users in their surroundings and to offer their services in an appropriate manner. Starting from a real application scenario, the usage of a robot as mobile information kiosk in a home store, some reliable methods for vision-based interaction, sound analysis and speech output have been developed. These methods are integrated into a prototypical interaction cycle that can be assumed as a general approach to human-machine interaction. Experimental results demonstrate the strengths and weaknesses of the proposed methods.