3GPP has specified a new radio interface, the Narrowband Internet of Things (NB-IoT), optimized for machine type traffic. It is kept simple in order to reduce device costs and to minimize battery consumption. It is also adapted to work in difficult radio conditions, which is a frequent operational area for certain machine type communication devices. Although NB-IoT is an independent radio interface, it is tightly connected with LTE, which also shows up in its integration in the current LTE specifications. This white paper introduces the NB-IoT technology with an emphasis on the tight connection to LTE.