The Internet-of-Things (IoT) revolution has triggered the need of massive connectivity for billions of devices requiring a system capacity which is far beyond the current network designs that can be supported. This emerging requirement has reshaped the society and industry in pursuing efficient communication paradigm. In particular, massive machine-type communications (mMTC) will be a prime driver for enabling the vision of scalable IoT with heterogeneous applications, where the massive access is of paramount importance. This book discusses important massive IoT scenarios and the key technical requirements of the corresponding massive access. We review the state-of-the-art IoT standards and mMTC solutions, and summarize the limitations of the existing schemes from the perspectives of the network architecture, random access procedure, and multiple access techniques. Here, we specify the massive access challenges and reveal that the facilitation of MTC invokes a dramatically different access scheme from current ones mainly designed for human-centric communications. Moreover, we propose several promising massive access solutions to overcome the limitations, where sufficient theoretical model and algorithm design guidance are provided. Besides, detailed simulation and engineering implementation methods are also included.