The fifth-generation (5G) wireless communications and networks are expected to significantly improve the data rate of the 4G systems. In one projection, a mobile user may enjoy the speed up to 10 Gb/s, with low latency, when the 5G communications becomes commercially available sometimes in the years 2020-2030. Given that 5G communications is still in its infancy, several issues remain open to researchers and require further investigation in order to realize this promising technology. One fundamental issue is time and frequency synchronization: given a unique architecture and channel characteristic of 5G communications, how does a receiver estimate the shifts in time and frequency from the received signal? This problem is important since synchronization is a prerequisite for reliable communication. On the other hand, synchronization is difficult since the numbers of transmit and receive antennas are expected to be enormous in 5G. This report reviews the characteristics of 5G communications, which include millimeter-wave transmission, massive multiple-input-multiple-output, and existing channel measurements, and analyzes the challenges that lie ahead in synchronization.