Laboratory rodents have been and will remain the most widely used model for studying mammalian physiology. Their availability, cost-effectiveness, ease in handling and breeding, similarity to human phylogeny have made rodents essential for biomedical experimentation. The findings of biomedical research based on these animals are used to estimate expected outcomes in humans. Besides the importance of meticulously understanding the biology and husbandry of rats and mice, it is very important to select the laboratory rodent of appropriate age for the specific experiment according to the age of the human being to which the outcome of that experiment aims to serve. This book is constructed to assist researchers in achieving greater research accuracy by providing a guide to correlate the age of laboratory rats or mice with that of humans at different stages of development throughout their respective life spans. The relation of the age of mice or rats with that of humans is derived using self-formulated calculations based on the duration of phases of life from birth to old age. The book also covers the basic biology and provides a brief overview of nurturing rats and mice in the laboratory. The goal of this book is to provide the researcher a go-to reference for easily and effectively selecting the best experimental rodent model.