The fast growing number of data centers in cloud raises high energy consumption problem. Thus there is a need for “Greening” in data center. ”Greening” means applying energy-saving techniques and designs in data center for power saving. Although cloud data centers nowadays are over-provisioned for high load, however, it is shown that the average data center load is only 30% [1]. There is lack of resource allocation mechanisms in data centers that caused the above problem. In this article, we review the “green” strategies, which are applied in data center to reduce energy consumption. First, we give the overview of the green metrics for data centers. Then, we present the most recent research in this area and give taxonomy of the related work.