Mobility as a Service (MaaS) is a new mobility concept gaining pace in many cities around the world. Its value proposition concerns integration of mobility services which is realized by providing trip planning and one-stop fare purchase for the user through a single platform. The MaaS vision is being enabled by advances in intelligent systems, the internet of things, cloud technology and big data management. This MaaS paper introduces and explains the MaaS concept (Chapter 1) and presents the MaaS enablers (Chapter 2 on Digital Enabler, Chapter 3 on Car-sharing and Chapter 4 on Bike-sharing) and MaaS application challenges (Chapter 5 on MaaS cost, risk and revenue challenges and Chapter 6 on MaaS infrastructure challenges). This paper was issued as Transport Trends and Economics 2018-2019 activity of the Economic Commission for Europe Working Party on Transport Trends and Economics