Hematology data from in vivo toxicology studies remains one of  the most predictive measures for human risk, as the same measurements made in pre-clinical toxicology studies can be made in early clinical trials. Covering the three main blood cell types – erythrocytes, leukocytes and thrombocytes, this work is designed to clarify topics for new entrants to the fields of laboratory animal hematology and toxicology, especially those topics where the rules for human hematology do not always apply. An entire chapter is devoted to immunotoxicology, a scientific discipline that is closely aligned with hematology. The text also addresses pre-analytical and analytical variables that affect animal studies, as these play a far more important part when interpreting data in contrast to humans where many of these variables can be well controlled or have less physiological effect. Information has been collated from published papers, textbooks and unpublished data: Readers are provided with key references to encourage further study