In cloud computing, in order to achieve resource sharing, virtual machines (VMs) of different tenants might be scheduled to run on the same physical machine, namely VMs co-residency, which would bring many new security issues. Therefore, security threats due to VMs co-residency, including resources interference, covert or side channel, denial of service and virtual machine load monitoring were reviewed in this paper. Besides, existing detection methods of co-residency were introduced, four kinds of defense about co-residency were summarized and further trends were also pointed out.