The world is experiencing demographic transition from higher level of fertility to lower level of fertility. However in Africa the pace of declining fertility is very low. This book analyse the low pace of declining fertility as the result of existence of son preference in Africa and Tanzania in particular. Parental preferences toward sons' influences the number of the children they could have, hence affecting the family size. This book use parity progression ratio to examine parents' behavior of continuing child bearing to search for a male offspring. Also, sex ratio at last birth was used to examine parental intention to stop childbearing after having at least one surviving son. The findings in this book show that parents' continue having children until they reached the desired number of sons. This book, therefore, provide an understanding of the relationship between son preference and family size. Furthermore, understanding of the relationship between son preference and family size are of great importance for the policy makers in order to deal adequately with the problems related to family size and population growth.