Wind energy, as a clean and renewable energy, is now being widely developed to reduce carbon dioxide production and mitigate the energy crisis. The urgent needs for wind energy motivate larger generators with lower cost, lower weight, and higher reliability. A popular solution is the direct-drive generator concept, such as a permanent magnet generator and superconducting (SC) generator. When referring to weight, volume, and cost, SC generators are superior to permanent magnet generators for wind turbines with rated power of 8 MW or more according to a report from the American National Renewable Energy Laboratory. In order to find out the suitable topology for megawatt-class direct-drive wind turbine generators, various designs of SC machines in literatures are carefully reviewed; advantages and disadvantages are discussed and a few ways to benefit from their advantages are pointed out. Electromagnetic, mechanical, and thermal structures, including excitation system, SC support system, cryogenic cooling system etc., are reviewed for wind SC machines. Design challenges and possible solutions are also summarized.