This chapter identifies specific background lesions in mice, rats, dogs, non-human primates, rabbits and minipigs. Background, incidental or spontaneous, lesions are pathology findings that are usually thought of as a change in tissue morphology outside of the range of normal variation for a particular species or strain. Common background lesions in non-human primates include pigmented macrophages in the small intestine and accessory splenic tissue in the pancreas. Anthracosis, or pneumoconiosis, is caused by the inhalation of atmospheric carbon particles by laboratory non-human primates housed near or within urban areas. The chapter discusses common causes of death or euthanasia of older rodents on carcinogenicity studies, as well as the significance of test article-related background findings. Animals that die during carcinogenicity studies must have a cause of death or demise attributed to them by the pathologist through light microscopic examination of the tissues.