Soil moisture is a key variable in water and energy exchanges in landatmosphere interface. The passive microwave remote sensing is the most potent technology to retrieve soil moisture. A brief introduction is made to microwave theory, and a general review of soil moisture retrieval algorithms is given. Three typical cases are illustrated based on the different microwave sensors by comparing various algorithms, which correspond to the threeparameter AMSRbased retrieval developed by Njoku and Li, the twoparameter SMMRbased retrieval developed by Owe et al. and the twoparameter SSM/Ibased retrieval developed by Wen et al. The insufficiency and potentials in the researches on soil moisture are discussed.