Abstract. The usage of public transportation system in the modern urban environment has changed in recent years. The residents of urban centers are increasingly turning into public transport as their preference, trying to reduce the burden on the road network and improve their quality of life. Public transportation evolves accordingly, new lines are created, old are getting extended and schedules alter in order to serve the growing traveling public. Finding our way from one region of the city to another has become a complex problem which requires a good knowledge of both the public transit and the road network to get solved. This need, combined with the introduction of technology and the Internet in our daily lives, has created an intense research activity in the field of urban transport and its efficient use. There have been developed intelligent routing algorithms which are based on open standards (Graphserver, OpenTripPlanner, GTFS, etc.) and provide reliable results optimizing the use of public transportation. The purpose of this thesis is to develop a mobile application for Android OS devices that, using existing data of urban transport in Athens and the already implemented server infrastructure of the OpenTripPlanner, will be able to provide the user of the mobile device with the optimal (less transit, faster) route between two points, using only public transport. Furthermore, using data saved locally on the device, the application will have the ability to present the timetable of urban transport and let the user explore lines and locate stops while minimizing network usage.