Book excerpt: Mycoplasmas are the smallest of free-living organisms and are intermediate between viruses and bacteria. Many species thrive as parasites in animal (including human) hosts. This book is based on proceedings of a conference held in Palermo, Italy. It reviews some of the most important mycoplasma diseases of sheep, goats and cattle including contagious bovine pleuropneumonia, contagious agalactia and calf pneumonia, which are listed by the OIE because of their economic implications.