As you know mastitis is a very costly disease on dairies. It is a complex, multi-factorial health problem and, all around the world, a large number of scientists, and their graduate students, are conducting research on this disease. For that reason, experts from more than 20 countries have decided to work together to produce this series of three MOOC, designed for graduate students, to give them the knowledge they need to initiate their research program. Of course, these MOOCs will also be useful for dairy practitioners, teachers, and for individuals that already have a solid scientific background, and are interested in learning about bovine mastitis. This second MOOC (of a duration of 18 hours), titled "Mastitis Epidemiology and Diagnostic" will discuss risk factors as well as measuring the prevalence and incidence of clinical and subclinical mastitis. This MOOC will also present the main pathogens causing the disease, their transmission modalities (contagious or environmental), the conventional diagnostic methods and the newest molecular avenues to identify them accurately. This series will be followed by a third MOOC that will focus on the control of mastitis. We hope this series will answer all your questions and will be useful for your professional development. Welcome to the "Mastitis epidemiology and diagnostic" MOOC!