In Release 13 the Third Generation Partnership Project (3GPP) developed the Narrowband Internet of Things (NB-IoT). It is a cellular technology providing wireless connectivity to machine-type services and applications. In 3GPP Releases 14 and 15, NB-IoT evolved to meet the requirements defined for a fifth generation (5G) communications system in 3GPP and in the International Telecommunications Union Radiocommunication (ITU-R) sector. This paper introduces NB IoT in the context of 3GPP's recent work on 5G. It presents the ITU-R International Mobile Telecommunication 2020 (IMT 2020) and 3GPP 5G performance requirements. A complete evaluation of the NB IoT performance for each of these requirements is presented, and it is shown that NB IoT meets the 5G performance requirements for massive IoT (mIoT) services.