This book discusses the basic knowledge about fertility in mammalian males and females. The authors listed and discussed the latest methods of fertility assessment such as; Computer-Assisted semen analysis 'CASA' and chromatin and DNA integrity. Discussed traditional methods are seminal plasma & sperm membrane proteins, sperm migration, sperm plasma membrane integrity, acrosomal integrity, capacitation status, sperm morphology, viability, sperm concentration, sperm motility, ejaculate volume, morphological and physiological characteristics of both sexes reproductive systems and their impact on fertility. Results on recent semen assessment and female fertility research on human populations, besides, many farm animals are presented. Also, taking a glimpse of history and steps of artificial insemination along with a brief overview of some genetic frontiers and management factors affecting fertility. This information is necessary for measuring fertility or semen quality.