The growing number, size, complexity and energy density of data centers due to increasing demand for storage, networking and computation bring a considerable energy challenge. Several measures to improve energy efficiency are being studied, not only to allow a supportable industry growth but also to reduce operational costs. Cooling energy consumption constitutes a large portion of the total consumption of data centers, which can account up to 40% in the case of inefficient cooling systems. In this paper a critical discussion on existing and emerging technologies for data center cooling systems was carried out. Fundamental aspects concerning advantages and drawbacks of each examined cooling system were discussed. Moreover a critical analysis on next future technology solutions for obtaining high energy efficiency data center is performed.