In the near future, there are expected to have at least billions of devices interconnected with each other. How to connect so many devices becomes a big issue. Machine-to-Machine (M2M) communications serve as the fundamental underlying technologies to support such Internet of Things (IoT) applications. The characteristics and services requirements of machine type communication devices (MTCDs) are totally different from the existing ones. Existing network technologies, ranging from personal area networks to wide area networks, are not well suited for M2M communications. Therefore, we first investigate the characteristics and service requirements for MTCDs. Recent advances in both cellular and capillary M2M communications are also discussed. Finally, we list some open issues and future research directions.