This study deals with the concept, rules and application of Qiyas as developed in the medieval period. The early chapters contain a general treatment of this principle. These are followed by chapters relating to definition of Qiyas, the original and the parallel case. The most difficult problem in Qiyas is the illah. We have extensively dealt with it dividing it into seven chapters. The Chapters relating to condition (shart), cause (sabab) and impediments (mawani') have also been included, as they are indirectly connected with the 'illah. As istihsan is a latent analogy (qiyas khafi), we have dealt with this problem in a separate chapter. Qiyas has remained all along a controversial subject. We have therefore analysed the points of view of its exponents and adversaries, and discussed their arguments in greater detail.