This book aims to give an idea about the genetic improvements in the most popular sheep breeds in Egypt, Rahmani and Ossimi. Recently, numbers of sheep population in Egypt have increased rapidly, reaching about more than 6 million head in 2013. Ossimi and Rahmani sheep represent about 35% of the total population of sheep in Egypt. Thus, it was necessary to investigate the possibility of genetically improving these two breeds. Estimation of genetic trends over time is problematic because of the difficulty to conduct experiments in uniform conditions over a period of several generations, so that changes in performance of a selected population may reflect, in part, both environmental and genetic changes. The objectives of the present work were to estimate additive genetic change and measure the phenotypic and environmental trends for lamb weight and ewe reproductive traits in two sheep flocks of Rahmani and Ossimi over a period of forty years.