Healthy research animals are both a prerequisite for meaningful research outcomes and an ethical obligation. Maintaining their health requires timely detection and appropriate treatment of disease. This, in turn, calls for all those involved in working with the animals, not only veterinarians, to possess a fundamental understanding of the species in their care. This chapter provides such essential knowledge for those working with nonhuman primates. It outlines the basic principles of blood analysis, radiology and physical examination, followed by an overview of the major organ systems, with a particular focus on macaque monkeys, and the diagnostic methods commonly used to assess them. The second part of the chapter focuses on monitoring the status of animals during anaesthesia, surgery and other procedures. Key techniques such as pulse oximetry, capnometry, blood pressure measurement, electrocardiography and temperature monitoring are presented, among others. Accurate diagnosis and continuous monitoring are crucial to ensuring animals’ well-being in both the short and long term, thereby safeguarding not only their health but also the quality and reliability of the scientific data obtained.