The meaning of quantitative precipitation is discussed. The instruments aboard Tropical Rainfall Measurement Mission (TRMM) and the products of the TRMM Microwave Imager from National Aeronautics and Space Administration are described. The principles of microwave precipitation retrieval, the comparison of physical methods and empirical methods are narrated, and some empirical algorithms are introduced and discussions are made on the effect of the tilted convective system on upwelling microwave brightness temperature, the clustering analysis algorithms and the neural network algorithms.