The housing conditions of laboratory rodents are an important factor that can affect their welfare, having behavioural, physiological and psychological effects on rodents. This raises both ethical and scientific concerns, particularly because laboratory rats constitute a major proportion of animals used in research. As such, their housing conditions should be improved. This book aims to provide, within the covers of one book, much of the information about the effect of the housing condition such as stocking density, cage enrichment and the time of delivering normal husbandry procedures on the welfare of laboratory rats. It involves a detailed description of ethogram of laboratory rats and the development of their behaviours. It also illustrates the importance of sleep behaviour as a new measure of welfare in laboratory rats. The work of this thesis was carried out as a part of a larger project investigating the proposals of the Council of Europe for housing newly weaned male laboratory rats. Therefore, it is hoped that what I have produced will not only be a source of information but, in many areas, it will be an enjoyable, educational read.