  Communicating on millimeter wave (mmWave) bands is ushering in a new epoch of
mobile communication which provides the availability of 10 Gbps high data rate
transmission. However, mmWave links are easily prone to short transmission
range communication because of the serious free space path loss and the
blockage by obstacles. To overcome these challenges, highly directional beams
are exploited to achieve robust links by hybrid beamforming. Accurately
aligning the transmitter and receiver beams, i.e. beam training, is vitally
important to high data rate transmission. However, it may cause huge overhead
which has negative effects on initial access, handover, and tracking. Besides,
the mobility patterns of users are complicated and dynamic, which may cause
tracking error and large tracking latency. An efficient beam tracking method
has a positive effect on sustaining robust links. This article provides an
overview of the beam training and tracking technologies on mmWave bands and
reveals the insights for future research in the 6th Generation (6G) mobile
network. Especially, some open research problems are proposed to realize fast,
accurate, and robust beam training and tracking. We hope that this survey
provides guidelines for the researchers in the area of mmWave communications.
